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Perl Documentation

NAME

perlop - Perl operators and precedence

DESCRIPTION

In Perl, the operator determines what operation is performed, independent of the type of the operands. For example pppppppppppppppppppppppppppppppppppppppppp is always a numeric addition, and if $x or $y do not contain numbers, an attempt is made to convert them to numbers first.

This is in contrast to many other dynamic languages, where the operation is determined by the type of the first argument. It also means that Perl has two versions of some operators, one for numeric and one for string comparison. For example pppppppppppppppppppppppppppppppppppppppppp compares two numbers for equality, and pppppppppppppppppppppppppppppppppppppppppp compares two strings.

There are a few exceptions though: x can be either string repetition or list repetition, depending on the type of the left operand, and &, |, ^ and ~ can be either string or numeric bit operations.

Operator Precedence and Associativity

Operator precedence and associativity work in Perl more or less like they do in mathematics.

Operator precedence means some operators are evaluated before others. For example, in pppppppppppppppppppppppppppppppppppppppppp, the multiplication has higher precedence so pppppppppppppppppppppppppppppppppppppppppp is evaluated first yielding pppppppppppppppppppppppppppppppppppppppppp and not pppppppppppppppppppppppppppppppppppppppppp.

Operator associativity defines what happens if a sequence of the same operators is used one after another: whether the evaluator will evaluate the left operations first, or the right first. For example, in pppppppppppppppppppppppppppppppppppppppppp, subtraction is left associative so Perl evaluates the expression left to right. pppppppppppppppppppppppppppppppppppppppppp is evaluated first making the expression pppppppppppppppppppppppppppppppppppppppppp and not pppppppppppppppppppppppppppppppppppppppppp.

Perl operators have the following associativity and precedence, listed from highest precedence to lowest. Operators borrowed from C keep the same precedence relationship with each other, even where C's precedence is slightly screwy. (This makes learning Perl easier for C folks.) With very few exceptions, these all operate on scalar values only, not array values.

left	terms and list operators (leftward)
left	->
nonassoc	++ --
right	**
right	! ~ \ and unary + and -
left	=~ !~
left	* / % x
left	+ - .
left	<< >>
nonassoc	named unary operators
nonassoc	< > <= >= lt gt le ge
nonassoc	== != <=> eq ne cmp ~~
left	&
left	| ^
left	&&
left	|| //
nonassoc	..  ...
right	?:
right	= += -= *= etc. goto last next redo dump
left	, =>
nonassoc	list operators (rightward)
right	not
left	and
left	or xor

In the following sections, these operators are covered in precedence order.

Many operators can be overloaded for objects. See overload.

Terms and List Operators (Leftward)

A TERM has the highest precedence in Perl. They include variables, quote and quote-like operators, any expression in parentheses, and any function whose arguments are parenthesized. Actually, there aren't really functions in this sense, just list operators and unary operators behaving as functions because you put parentheses around the arguments. These are all documented in perlfunc.

If any list operator (print(), etc.) or any unary operator (chdir(), etc.) is followed by a left parenthesis as the next token, the operator and arguments within parentheses are taken to be of highest precedence, just like a normal function call.

In the absence of parentheses, the precedence of list operators such as print, sort, or chmod is either very high or very low depending on whether you are looking at the left side or the right side of the operator. For example, in

@ary = (1, 3, sort 4, 2);
print @ary;		# prints 1324

the commas on the right of the sort are evaluated before the sort, but the commas on the left are evaluated after. In other words, list operators tend to gobble up all arguments that follow, and then act like a simple TERM with regard to the preceding expression. Be careful with parentheses:

# These evaluate exit before doing the print:
print($foo, exit);	# Obviously not what you want.
print $foo, exit;	# Nor is this.
# These do the print before evaluating exit:
(print $foo), exit;	# This is what you want.
print($foo), exit;	# Or this.
print ($foo), exit;	# Or even this.

Also note that

print ($foo & 255) + 1, "\n";

probably doesn't do what you expect at first glance. The parentheses enclose the argument list for print which is evaluated (printing the result of pppppppppppppppppppppppppppppppppppppppppp). Then one is added to the return value of print (usually 1). The result is something like this:

1 + 1, "\n";    # Obviously not what you meant.

To do what you meant properly, you must write:

print(($foo & 255) + 1, "\n");

See "Named Unary Operators" for more discussion of this.

Also parsed as terms are the pppppppppppppppppppppppppppppppppppppppppp and pppppppppppppppppppppppppppppppppppppppppp constructs, as well as subroutine and method calls, and the anonymous constructors [] and {}.

See also "Quote and Quote-like Operators" toward the end of this section, as well as "I/O Operators".

The Arrow Operator

"->" is an infix dereference operator, just as it is in C and C++. If the right side is either a [...], {...}, or a (...) subscript, then the left side must be either a hard or symbolic reference to an array, a hash, or a subroutine respectively. (Or technically speaking, a location capable of holding a hard reference, if it's an array or hash reference being used for assignment.) See perlreftut and perlref.

Otherwise, the right side is a method name or a simple scalar variable containing either the method name or a subroutine reference, and the left side must be either an object (a blessed reference) or a class name (that is, a package name). See perlobj.

The dereferencing cases (as opposed to method-calling cases) are somewhat extended by the postderef feature. For the details of that feature, consult "Postfix Dereference Syntax" in perlref.

Auto-increment and Auto-decrement

"++" and "--" work as in C. That is, if placed before a variable, they increment or decrement the variable by one before returning the value, and if placed after, increment or decrement after returning the value.

$i = 0;  $j = 0;
print $i++;  # prints 0
print ++$j;  # prints 1

Note that just as in C, Perl doesn't define when the variable is incremented or decremented. You just know it will be done sometime before or after the value is returned. This also means that modifying a variable twice in the same statement will lead to undefined behavior. Avoid statements like:

$i = $i ++;
print ++ $i + $i ++;

Perl will not guarantee what the result of the above statements is.

The auto-increment operator has a little extra builtin magic to it. If you increment a variable that is numeric, or that has ever been used in a numeric context, you get a normal increment. If, however, the variable has been used in only string contexts since it was set, and has a value that is not the empty string and matches the pattern /^[a-zA-Z]*[0-9]*\z/, the increment is done as a string, preserving each character within its range, with carry:

print ++($foo = "99");	# prints "100"
print ++($foo = "a0");	# prints "a1"
print ++($foo = "Az");	# prints "Ba"
print ++($foo = "zz");	# prints "aaa"

undef is always treated as numeric, and in particular is changed to 0 before incrementing (so that a post-increment of an undef value will return 0 rather than undef).

The auto-decrement operator is not magical.

Exponentiation

Binary "**" is the exponentiation operator. It binds even more tightly than unary minus, so -2**4 is -(2**4), not (-2)**4. (This is implemented using C's pow(3) function, which actually works on doubles internally.)

Note that certain exponentiation expressions are ill-defined: these include 0**0, 1**Inf, and Inf**0. Do not expect any particular results from these special cases, the results are platform-dependent.

Symbolic Unary Operators

Unary "!" performs logical negation, that is, "not". See also not for a lower precedence version of this.

Unary "-" performs arithmetic negation if the operand is numeric, including any string that looks like a number. If the operand is an identifier, a string consisting of a minus sign concatenated with the identifier is returned. Otherwise, if the string starts with a plus or minus, a string starting with the opposite sign is returned. One effect of these rules is that -bareword is equivalent to the string "-bareword". If, however, the string begins with a non-alphabetic character (excluding "+" or "-"), Perl will attempt to convert the string to a numeric, and the arithmetic negation is performed. If the string cannot be cleanly converted to a numeric, Perl will give the warning Argument "the string" isn't numeric in negation (-) at ....

Unary "~" performs bitwise negation, that is, 1's complement. For example, pppppppppppppppppppppppppppppppppppppppppp is 0640. (See also "Integer Arithmetic" and "Bitwise String Operators".) Note that the width of the result is platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64 bits wide on a 64-bit platform, so if you are expecting a certain bit width, remember to use the "&" operator to mask off the excess bits.

When complementing strings, if all characters have ordinal values under 256, then their complements will, also. But if they do not, all characters will be in either 32- or 64-bit complements, depending on your architecture. So for example, ~"\x{3B1}" is "\x{FFFF_FC4E}" on 32-bit machines and "\x{FFFF_FFFF_FFFF_FC4E}" on 64-bit machines.

If the experimental "bitwise" feature is enabled via pppppppppppppppppppppppppppppppppppppppppp, then unary "~" always treats its argument as a number, and an alternate form of the operator, "~.", always treats its argument as a string. So ~0 and ~"0" will both give 2**32-1 on 32-bit platforms, whereas ~.0 and ~."0" will both yield "\xff". This feature produces a warning unless you use pppppppppppppppppppppppppppppppppppppppppp.

Unary "+" has no effect whatsoever, even on strings. It is useful syntactically for separating a function name from a parenthesized expression that would otherwise be interpreted as the complete list of function arguments. (See examples above under "Terms and List Operators (Leftward)".)

Unary "\" creates a reference to whatever follows it. See perlreftut and perlref. Do not confuse this behavior with the behavior of backslash within a string, although both forms do convey the notion of protecting the next thing from interpolation.

Binding Operators

Binary "=~" binds a scalar expression to a pattern match. Certain operations search or modify the string $_ by default. This operator makes that kind of operation work on some other string. The right argument is a search pattern, substitution, or transliteration. The left argument is what is supposed to be searched, substituted, or transliterated instead of the default $_. When used in scalar context, the return value generally indicates the success of the operation. The exceptions are substitution (s///) and transliteration (y///) with the /r (non-destructive) option, which cause the return value to be the result of the substitution. Behavior in list context depends on the particular operator. See "Regexp Quote-Like Operators" for details and perlretut for examples using these operators.

If the right argument is an expression rather than a search pattern, substitution, or transliteration, it is interpreted as a search pattern at run time. Note that this means that its contents will be interpolated twice, so

'\\' =~ q'\\';

is not ok, as the regex engine will end up trying to compile the pattern \, which it will consider a syntax error.

Binary "!~" is just like "=~" except the return value is negated in the logical sense.

Binary "!~" with a non-destructive substitution (s///r) or transliteration (y///r) is a syntax error.

Multiplicative Operators

Binary "*" multiplies two numbers.

Binary "/" divides two numbers.

Binary "%" is the modulo operator, which computes the division remainder of its first argument with respect to its second argument. Given integer operands $m and $n: If $n is positive, then pppppppppppppppppppppppppppppppppppppppppp is $m minus the largest multiple of $n less than or equal to $m. If $n is negative, then pppppppppppppppppppppppppppppppppppppppppp is $m minus the smallest multiple of $n that is not less than $m (that is, the result will be less than or equal to zero). If the operands $m and $n are floating point values and the absolute value of $n (that is abs($n)) is less than pppppppppppppppppppppppppppppppppppppppppp, only the integer portion of $m and $n will be used in the operation (Note: here UV_MAX means the maximum of the unsigned integer type). If the absolute value of the right operand (abs($n)) is greater than or equal to pppppppppppppppppppppppppppppppppppppppppp, "%" computes the floating-point remainder $r in the equation pppppppppppppppppppppppppppppppppppppppppp where $i is a certain integer that makes $r have the same sign as the right operand $n (not as the left operand $m like C function fmod()) and the absolute value less than that of $n. Note that when pppppppppppppppppppppppppppppppppppppppppp is in scope, "%" gives you direct access to the modulo operator as implemented by your C compiler. This operator is not as well defined for negative operands, but it will execute faster.

Binary "x" is the repetition operator. In scalar context or if the left operand is not enclosed in parentheses, it returns a string consisting of the left operand repeated the number of times specified by the right operand. In list context, if the left operand is enclosed in parentheses or is a list formed by qw/STRING/, it repeats the list. If the right operand is zero or negative (raising a warning on negative), it returns an empty string or an empty list, depending on the context.

print '-' x 80;		# print row of dashes
print "\t" x ($tab/8), ' ' x ($tab%8);	# tab over
@ones = (1) x 80;		# a list of 80 1's
@ones = (5) x @ones;	# set all elements to 5

Additive Operators

Binary "+" returns the sum of two numbers.

Binary "-" returns the difference of two numbers.

Binary "." concatenates two strings.

Shift Operators

Binary "<<" returns the value of its left argument shifted left by the number of bits specified by the right argument. Arguments should be integers. (See also "Integer Arithmetic".)

Binary ">>" returns the value of its left argument shifted right by the number of bits specified by the right argument. Arguments should be integers. (See also "Integer Arithmetic".)

If pppppppppppppppppppppppppppppppppppppppppp (see "Integer Arithmetic") is in force then signed C integers are used (arithmetic shift), otherwise unsigned C integers are used (logical shift), even for negative shiftees. In arithmetic right shift the sign bit is replicated on the left, in logical shift zero bits come in from the left.

Either way, the implementation isn't going to generate results larger than the size of the integer type Perl was built with (32 bits or 64 bits).

Shifting by negative number of bits means the reverse shift: left shift becomes right shift, right shift becomes left shift. This is unlike in C, where negative shift is undefined.

Shifting by more bits than the size of the integers means most of the time zero (all bits fall off), except that under pppppppppppppppppppppppppppppppppppppppppp right overshifting a negative shiftee results in -1. This is unlike in C, where shifting by too many bits is undefined. A common C behavior is "shift by modulo wordbits", so that for example

1 >> 64 == 1 >> (64 % 64) == 1 >> 0 == 1  # Common C behavior.

but that is completely accidental.

If you get tired of being subject to your platform's native integers, the pppppppppppppppppppppppppppppppppppppppppp pragma neatly sidesteps the issue altogether:

print 20 << 20;  # 20971520
print 20 << 40;  # 5120 on 32-bit machines, 
                 # 21990232555520 on 64-bit machines
use bigint;
print 20 << 100; # 25353012004564588029934064107520

Named Unary Operators

The various named unary operators are treated as functions with one argument, with optional parentheses.

If any list operator (print(), etc.) or any unary operator (chdir(), etc.) is followed by a left parenthesis as the next token, the operator and arguments within parentheses are taken to be of highest precedence, just like a normal function call. For example, because named unary operators are higher precedence than ||:

chdir $foo    || die;	# (chdir $foo) || die
chdir($foo)   || die;	# (chdir $foo) || die
chdir ($foo)  || die;	# (chdir $foo) || die
chdir +($foo) || die;	# (chdir $foo) || die

but, because "*" is higher precedence than named operators:

chdir $foo * 20;	# chdir ($foo * 20)
chdir($foo) * 20;	# (chdir $foo) * 20
chdir ($foo) * 20;	# (chdir $foo) * 20
chdir +($foo) * 20;	# chdir ($foo * 20)
rand 10 * 20;	# rand (10 * 20)
rand(10) * 20;	# (rand 10) * 20
rand (10) * 20;	# (rand 10) * 20
rand +(10) * 20;	# rand (10 * 20)

Regarding precedence, the filetest operators, like -f, -M, etc. are treated like named unary operators, but they don't follow this functional parenthesis rule. That means, for example, that -f($file).".bak" is equivalent to pppppppppppppppppppppppppppppppppppppppppp.

See also "Terms and List Operators (Leftward)".

Relational Operators

Perl operators that return true or false generally return values that can be safely used as numbers. For example, the relational operators in this section and the equality operators in the next one return 1 for true and a special version of the defined empty string, "", which counts as a zero but is exempt from warnings about improper numeric conversions, just as pppppppppppppppppppppppppppppppppppppppppp is.

Binary "<" returns true if the left argument is numerically less than the right argument.

Binary ">" returns true if the left argument is numerically greater than the right argument.

Binary "<=" returns true if the left argument is numerically less than or equal to the right argument.

Binary ">=" returns true if the left argument is numerically greater than or equal to the right argument.

Binary "lt" returns true if the left argument is stringwise less than the right argument.

Binary "gt" returns true if the left argument is stringwise greater than the right argument.

Binary "le" returns true if the left argument is stringwise less than or equal to the right argument.

Binary "ge" returns true if the left argument is stringwise greater than or equal to the right argument.

Equality Operators

Binary "==" returns true if the left argument is numerically equal to the right argument.

Binary "!=" returns true if the left argument is numerically not equal to the right argument.

Binary "<=>" returns -1, 0, or 1 depending on whether the left argument is numerically less than, equal to, or greater than the right argument. If your platform supports NaN's (not-a-numbers) as numeric values, using them with "<=>" returns undef. NaN is not "<", "==", ">", "<=" or ">=" anything (even NaN), so those 5 return false. pppppppppppppppppppppppppppppppppppppppppp returns true, as does ppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppppp. If your platform doesn't support NaN's then NaN is just a string with numeric value 0.

$ perl -le '$x = "NaN"; print "No NaN support here" if $x == $x'
$ perl -le '$x = "NaN"; print "NaN support here" if $x != $x'

(Note that the bigint, bigrat, and bignum pragmas all support "NaN".)

Binary "eq" returns true if the left argument is stringwise equal to the right argument.

Binary "ne" returns true if the left argument is stringwise not equal to the right argument.

Binary "cmp" returns -1, 0, or 1 depending on whether the left argument is stringwise less than, equal to, or greater than the right argument.

Binary "~~" does a smartmatch between its arguments. Smart matching is described in the next section.

"lt", "le", "ge", "gt" and "cmp" use the collation (sort) order specified by the current LC_COLLATE locale if a pppppppppppppppppppppppppppppppppppppppppp form that includes collation is in effect. See perllocale. Do not mix these with Unicode, only use them with legacy 8-bit locale encodings. The standard Unicode::Collate and Unicode::Collate::Locale modules offer much more powerful solutions to collation issues.

For case-insensitive comparisions, look at the "fc" in perlfunc case-folding function, available in Perl v5.16 or later:

if ( fc($x) eq fc($y) ) { ... }

Smartmatch Operator

First available in Perl 5.10.1 (the 5.10.0 version behaved differently), binary ~~ does a "smartmatch" between its arguments. This is mostly used implicitly in the when construct described in perlsyn, although not all when clauses call the smartmatch operator. Unique among all of Perl's operators, the smartmatch operator can recurse. The smartmatch operator is experimental and its behavior is subject to change.

It is also unique in that all other Perl operators impose a context (usually string or numeric context) on their operands, autoconverting those operands to those imposed contexts. In contrast, smartmatch infers contexts from the actual types of its operands and uses that type information to select a suitable comparison mechanism.

The ~~ operator compares its operands "polymorphically", determining how to compare them according to their actual types (numeric, string, array, hash, etc.) Like the equality operators with which it shares the same precedence, ~~ returns 1 for true and "" for false. It is often best read aloud as "in", "inside of", or "is contained in", because the left operand is often looked for inside the right operand. That makes the order of the operands to the smartmatch operand often opposite that of the regular match operator. In other words, the "smaller" thing is usually placed in the left operand and the larger one in the right.

The behavior of a smartmatch depends on what type of things its arguments are, as determined by the following table. The first row of the table whose types apply determines the smartmatch behavior. Because what actually happens is mostly determined by the type of the second operand, the table is sorted on the right operand instead of on the left.

Left      Right      Description and pseudocode                               
===============================================================
Any       undef      check whether Any is undefined                    
               like: !defined Any
Any       Object     invoke ~~ overloading on Object, or die
Right operand is an ARRAY:
Left      Right      Description and pseudocode                               
===============================================================
ARRAY1    ARRAY2     recurse on paired elements of ARRAY1 and ARRAY2[2]
               like: (ARRAY1[0] ~~ ARRAY2[0])
                       && (ARRAY1[1] ~~ ARRAY2[1]) && ...
HASH      ARRAY      any ARRAY elements exist as HASH keys             
               like: grep { exists HASH->{$_} } ARRAY
Regexp    ARRAY      any ARRAY elements pattern match Regexp
               like: grep { /Regexp/ } ARRAY
undef     ARRAY      undef in ARRAY                                    
               like: grep { !defined } ARRAY
Any       ARRAY      smartmatch each ARRAY element[3]                   
               like: grep { Any ~~ $_ } ARRAY
Right operand is a HASH:
Left      Right      Description and pseudocode                               
===============================================================
HASH1     HASH2      all same keys in both HASHes                      
               like: keys HASH1 ==
                        grep { exists HASH2->{$_} } keys HASH1
ARRAY     HASH       any ARRAY elements exist as HASH keys             
               like: grep { exists HASH->{$_} } ARRAY
Regexp    HASH       any HASH keys pattern match Regexp                
               like: grep { /Regexp/ } keys HASH
undef     HASH       always false (undef can't be a key)               
               like: 0 == 1
Any       HASH       HASH key existence                                
               like: exists HASH->{Any}
Right operand is CODE:
Left      Right      Description and pseudocode                               
===============================================================
ARRAY     CODE       sub returns true on all ARRAY elements[1]
               like: !grep { !CODE->($_) } ARRAY
HASH      CODE       sub returns true on all HASH keys[1]
               like: !grep { !CODE->($_) } keys HASH
Any       CODE       sub passed Any returns true              
               like: CODE->(Any)

Right operand is a Regexp:

Left      Right      Description and pseudocode                               
===============================================================
ARRAY     Regexp     any ARRAY elements match Regexp                   
               like: grep { /Regexp/ } ARRAY
HASH      Regexp     any HASH keys match Regexp                        
               like: grep { /Regexp/ } keys HASH
Any       Regexp     pattern match                                     
               like: Any =~ /Regexp/
Other:
Left      Right      Description and pseudocode                               
===============================================================
Object    Any        invoke ~~ overloading on Object,
                     or fall back to...
Any       Num        numeric equality                                  
                like: Any == Num
Num       nummy[4]    numeric equality
                like: Num == nummy
undef     Any        check whether undefined
                like: !defined(Any)
Any       Any        string equality                                   
                like: Any eq Any

Notes:

  1. Empty hashes or arrays match.
  2. That is, each element smartmatches the element of the same index in the other array.[3]
  3. If a circular reference is found, fall back to referential equality.
  4. Either an actual number, or a string that looks like one.

The smartmatch implicitly dereferences any non-blessed hash or array reference, so the HASH and ARRAY entries apply in those cases. For blessed references, the Object entries apply. Smartmatches involving hashes only consider hash keys, never hash values.

The "like" code entry is not always an exact rendition. For example, the smartmatch operator short-circuits whenever possible, but grep does not. Also, grep in scalar context returns the number of matches, but ~~ returns only true or false.

Unlike most operators, the smartmatch operator knows to treat undef specially:

use v5.10.1;
@array = (1, 2, 3, undef, 4, 5);
say "some elements undefined" if undef ~~ @array;

Each operand is considered in a modified scalar context, the modification being that array and hash variables are passed by reference to the operator, which implicitly dereferences them. Both elements of each pair are the same:

use v5.10.1;
my %hash = (red    => 1, blue   => 2, green  => 3,
            orange => 4, yellow => 5, purple => 6,
            black  => 7, grey   => 8, white  => 9);
my @array = qw(red blue green);
say "some array elements in hash keys" if  @array ~~  %hash;
say "some array elements in hash keys" if \@array ~~ \%hash;
say "red in array" if "red" ~~  @array;
say "red in array" if "red" ~~ \@array;
say "some keys end in e" if /e$/ ~~  %hash;
say "some keys end in e" if /e$/ ~~ \%hash;

Two arrays smartmatch if each element in the first array smartmatches (that is, is "in") the corresponding element in the second array, recursively.

use v5.10.1;
my @little = qw(red blue green);
my @bigger = ("red", "blue", [ "orange", "green" ] );
if (@little ~~ @bigger) {  # true!
    say "little is contained in bigger";
}

Because the smartmatch operator recurses on nested arrays, this will still report that "red" is in the array.

use v5.10.1;
my @array = qw(red blue green);
my $nested_array = [[[[[[[ @array ]]]]]]];
say "red in array" if "red" ~~ $nested_array;

If two arrays smartmatch each other, then they are deep copies of each others' values, as this example reports:

use v5.12.0;
my @a = (0, 1, 2, [3, [4, 5], 6], 7); 
my @b = (0, 1, 2, [3, [4, 5], 6], 7);
if (@a ~~ @b && @b ~~ @a) {
    say "a and b are deep copies of each other";
} 
elsif (@a ~~ @b) {
    say "a smartmatches in b";
} 
elsif (@b ~~ @a) {
    say "b smartmatches in a";
} 
else {
    say "a and b don't smartmatch each other at all";
}

If you were to set pppppppppppppppppppppppppppppppppppppppppp, then instead of reporting that "a and b are deep copies of each other", it now reports that "b smartmatches in a". That's because the corresponding position in @a contains an array that (eventually) has a 4 in it.

Smartmatching one hash against another reports whether both contain the same keys, no more and no less. This could be used to see whether two records have the same field names, without caring what values those fields might have. For example:

use v5.10.1;
sub make_dogtag {
    state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };
my ($class, $init_fields) = @_;
die "Must supply (only) name, rank, and serial number"
    unless $init_fields ~~ $REQUIRED_FIELDS;
    ...
}

However, this only does what you mean if $init_fields is indeed a hash reference. The condition $init_fields ~~ $REQUIRED_FIELDS also allows the strings "name", "rank", "serial_num" as well as any array reference that contains "name" or "rank" or "serial_num" anywhere to pass through.

The smartmatch operator is most often used as the implicit operator of a when clause. See the section on "Switch Statements" in perlsyn.

Smartmatching of Objects

To avoid relying on an object's underlying representation, if the smartmatch's right operand is an object that doesn't overload ~~, it raises the exception "Smartmatching a non-overloaded object breaks encapsulation". That's because one has no business digging around to see whether something is "in" an object. These are all illegal on objects without a ~~ overload:

 %hash ~~ $object
    42 ~~ $object
"fred" ~~ $object

However, you can change the way an object is smartmatched by overloading the ~~ operator. This is allowed to extend the usual smartmatch semantics. For objects that do have an ~~ overload, see overload.

Using an object as the left operand is allowed, although not very useful. Smartmatching rules take precedence over overloading, so even if the object in the left operand has smartmatch overloading, this will be ignored. A left operand that is a non-overloaded object falls back on a string or numeric comparison of whatever the ref operator returns. That means that

$object ~~ X

does not invoke the overload method with X as an argument. Instead the above table is consulted as normal, and based on the type of X, overloading may or may not be invoked. For simple strings or numbers, "in" becomes equivalent to this:

$object ~~ $number          ref($object) == $number
$object ~~ $string          ref($object) eq $string

For example, this reports that the handle smells IOish (but please don't really do this!):

use IO::Handle;
my $fh = IO::Handle->new();
if ($fh ~~ /\bIO\b/) {
    say "handle smells IOish";
}

That's because it treats $fh as a string like "IO::Handle=GLOB(0x8039e0)", then pattern matches against that.

Bitwise And

Binary "&" returns its operands ANDed together bit by bit. Although no warning is currently raised, the result is not well defined when this operation is performed on operands that aren't either numbers (see "Integer Arithmetic") nor bitstrings (see "Bitwise String Operators").

Note that "&" has lower priority than relational operators, so for example the parentheses are essential in a test like

print "Even\n" if ($x & 1) == 0;

If the experimental "bitwise" feature is enabled via pppppppppppppppppppppppppppppppppppppppppp, then this operator always treats its operand as numbers. This feature produces a warning unless you also use ppppppppppppppppppppppppppppppppppp.

Bitwise Or and Exclusive Or

Binary "|" returns its operands ORed together bit by bit.

Binary "^" returns its operands XORed together bit by bit.

Although no warning is currently raised, the results are not well defined when these operations are performed on operands that aren't either numbers (see "Integer Arithmetic") nor bitstrings (see ""Bitwise String Operators"").

Note that "|" and "^" have lower priority than relational operators, so for example the parentheses are essential in a test like

print "false\n" if (8 | 2) != 10;

If the experimental "bitwise" feature is enabled via pppppppppppppppppppppppppppppppppppppppppp, then this operator always treats its operand as numbers. This feature produces a warning unless you also use pppppppppppppppppppppppppppppppppppppppppp.

C-style Logical And

Binary "&&" performs a short-circuit logical AND operation. That is, if the left operand is false, the right operand is not even evaluated. Scalar or list context propagates down to the right operand if it is evaluated.

C-style Logical Or

Binary "||" performs a short-circuit logical OR operation. That is, if the left operand is true, the right operand is not even evaluated. Scalar or list context propagates down to the right operand if it is evaluated.

Logical Defined-Or

Although it has no direct equivalent in C, Perl's // operator is related to its C-style "or". In fact, it's exactly the same as ||, except that it tests the left hand side's definedness instead of its truth. Thus, pppppppppppppppppppppppppppppppppppppppppp returns the value of EXPR1 if it's defined, otherwise, the value of EXPR2 is returned. (EXPR1 is evaluated in scalar context, EXPR2 in the context of // itself). Usually, this is the same result as pppppppppppppppppppppppppppppppppppppppppp (except that the ternary-operator form can be used as a lvalue, while pppppppppppppppppppppppppppppppppppppppppp cannot). This is very useful for providing default values for variables. If you actually want to test if at least one of $x and $y is defined, use pppppppppppppppppppppppppppppppppppppppppp.

The ||, // and && operators return the last value evaluated (unlike C's || and &&, which return 0 or 1). Thus, a reasonably portable way to find out the home directory might be:

$home =  $ENV{HOME}
	  // $ENV{LOGDIR}
	  // (getpwuid($<))[7]
	  // die "You're homeless!\n";

In particular, this means that you shouldn't use this for selecting between two aggregates for assignment:

@a = @b || @c;            # This doesn't do the right thing
@a = scalar(@b) || @c;    # because it really means this.
@a = @b ? @b : @c;        # This works fine, though.

As alternatives to && and || when used for control flow, Perl provides the and and or operators (see below). The short-circuit behavior is identical. The precedence of "and" and "or" is much lower, however, so that you can safely use them after a list operator without the need for parentheses:

unlink "alpha", "beta", "gamma"
	    or gripe(), next LINE;

With the C-style operators that would have been written like this:

unlink("alpha", "beta", "gamma")
	    || (gripe(), next LINE);

It would be even more readable to write that this way:

unless(unlink("alpha", "beta", "gamma")) {
    gripe();
    next LINE;
}

Using "or" for assignment is unlikely to do what you want; see below.

Range Operators

Binary ".." is the range operator, which is really two different operators depending on the context. In list context, it returns a list of values counting (up by ones) from the left value to the right value. If the left value is greater than the right value then it returns the empty list. The range operator is useful for writing pppppppppppppppppppppppppppppppppppppppppp loops and for doing slice operations on arrays. In the current implementation, no temporary array is created when the range operator is used as the expression in foreach loops, but older versions of Perl might burn a lot of memory when you write something like this:

for (1 .. 1_000_000) {
	# code
}

The range operator also works on strings, using the magical auto-increment, see below.

In scalar context, ".." returns a boolean value. The operator is bistable, like a flip-flop, and emulates the line-range (comma) operator of sed, awk, and various editors. Each ".." operator maintains its own boolean state, even across calls to a subroutine that contains it. It is false as long as its left operand is false. Once the left operand is true, the range operator stays true until the right operand is true, AFTER which the range operator becomes false again. It doesn't become false till the next time the range operator is evaluated. It can test the right operand and become false on the same evaluation it became true (as in awk), but it still returns true once. If you don't want it to test the right operand until the next evaluation, as in sed, just use three dots ("...") instead of two. In all other regards, "..." behaves just like ".." does.

The right operand is not evaluated while the operator is in the "false" state, and the left operand is not evaluated while the operator is in the "true" state. The precedence is a little lower than || and &&. The value returned is either the empty string for false, or a sequence number (beginning with 1) for true. The sequence number is reset for each range encountered. The final sequence number in a range has the string "E0" appended to it, which doesn't affect its numeric value, but gives you something to search for if you want to exclude the endpoint. You can exclude the beginning point by waiting for the sequence number to be greater than 1.

If either operand of scalar ".." is a constant expression, that operand is considered true if it is equal (==) to the current input line number (the $. variable).

To be pedantic, the comparison is actually pppppppppppppppppppppppppppppppppppppppppp, but that is only an issue if you use a floating point expression; when implicitly using $. as described in the previous paragraph, the comparison is pppppppppppppppppppppppppppppppppppppppppp which is only an issue when $. is set to a floating point value and you are not reading from a file. Furthermore, pppppppppppppppppppppppppppppppppppppppppp or pppppppppppppppppppppppppppppppppppppppppp will not do what you want in scalar context because each of the operands are evaluated using their integer representation.

Examples:

As a scalar operator:

if (101 .. 200) { print; } # print 2nd hundred lines, short for
                           #  if ($. == 101 .. $. == 200) { print; }
next LINE if (1 .. /^$/);  # skip header lines, short for
                           #   next LINE if ($. == 1 .. /^$/);
                           # (typically in a loop labeled LINE)
s/^/> / if (/^$/ .. eof());  # quote body
# parse mail messages
while (<>) {
    $in_header =   1  .. /^$/;
    $in_body   = /^$/ .. eof;
    if ($in_header) {
        # do something
    } else { # in body
        # do something else
    }
} continue {
    close ARGV if eof;             # reset $. each file
}

Here's a simple example to illustrate the difference between the two range operators:

@lines = ("   - Foo",
          "01 - Bar",
          "1  - Baz",
          "   - Quux");
foreach (@lines) {
    if (/0/ .. /1/) {
        print "$_\n";
    }
}

This program will print only the line containing "Bar". If the range operator is changed to ..., it will also print the "Baz" line.

And now some examples as a list operator:

for (101 .. 200) { print }      # print $_ 100 times
@foo = @foo[0 .. $#foo];        # an expensive no-op
@foo = @foo[$#foo-4 .. $#foo];  # slice last 5 items

The range operator (in list context) makes use of the magical auto-increment algorithm if the operands are strings. You can say

@alphabet = ("A" .. "Z");

to get all normal letters of the English alphabet, or

$hexdigit = (0 .. 9, "a" .. "f")[$num & 15];

to get a hexadecimal digit, or

@z2 = ("01" .. "31");
print $z2[$mday];

to get dates with leading zeros.

If the final value specified is not in the sequence that the magical increment would produce, the sequence goes until the next value would be longer than the final value specified.

If the initial value specified isn't part of a magical increment sequence (that is, a non-empty string matching /^[a-zA-Z]*[0-9]*\z/), only the initial value will be returned. So the following will only return an alpha:

use charnames "greek";
my @greek_small =  ("\N{alpha}" .. "\N{omega}");

To get the 25 traditional lowercase Greek letters, including both sigmas, you could use this instead:

use charnames "greek";
my @greek_small =  map { chr } ( ord("\N{alpha}") 
                                    ..
                                 ord("\N{omega}") 
                               );

However, because there are many other lowercase Greek characters than just those, to match lowercase Greek characters in a regular expression, you could use the pattern /(?:(?=\p{Greek})\p{Lower})+/ (or the "experimental feature" ppppppppppppppppppppppppppppppp).

Because each operand is evaluated in integer form, pppppppppppppppppppppppppppppppppppppppppp will return two elements in list context.

@list = (2.18 .. 3.14); # same as @list = (2 .. 3);

Conditional Operator

Ternary "?:" is the conditional operator, just as in C. It works much like an if-then-else. If the argument before the ? is true, the argument before the : is returned, otherwise the argument after the : is returned. For example:

printf "I have %d dog%s.\n", $n,
	    ($n == 1) ? "" : "s";

Scalar or list context propagates downward into the 2nd or 3rd argument, whichever is selected.

$x = $ok ? $y : $z;  # get a scalar
@x = $ok ? @y : @z;  # get an array
$x = $ok ? @y : @z;  # oops, that's just a count!

The operator may be assigned to if both the 2nd and 3rd arguments are legal lvalues (meaning that you can assign to them):

($x_or_y ? $x : $y) = $z;

Because this operator produces an assignable result, using assignments without parentheses will get you in trouble. For example, this:

$x % 2 ? $x += 10 : $x += 2

Really means this:

(($x % 2) ? ($x += 10) : $x) += 2

Rather than this:

($x % 2) ? ($x += 10) : ($x += 2)

That should probably be written more simply as:

$x += ($x % 2) ? 10 : 2;

Assignment Operators

"=" is the ordinary assignment operator.

Assignment operators work as in C. That is,

$x += 2;

is equivalent to

$x = $x + 2;

although without duplicating any side effects that dereferencing the lvalue might trigger, such as from tie(). Other assignment operators work similarly. The following are recognized:

**=    +=    *=    &=    &.=    <<=    &&=
       -=    /=    |=    |.=    >>=    ||=
       .=    %=    ^=    ^.=           //=
             x=

Although these are grouped by family, they all have the precedence of assignment. These combined assignment operators can only operate on scalars, whereas the ordinary assignment operator can assign to arrays, hashes, lists and even references. (See "Context" and "List value constructors" in perldata, and "Assigning to References>" in perlref.)

Unlike in C, the scalar assignment operator produces a valid lvalue. Modifying an assignment is equivalent to doing the assignment and then modifying the variable that was assigned to. This is useful for modifying a copy of something, like this:

($tmp = $global) =~ tr/13579/24680/;

Although as of 5.14, that can be also be accomplished this way:

use v5.14;
$tmp = ($global =~  tr/13579/24680/r);

Likewise,

($x += 2) *= 3;

is equivalent to

$x += 2;
$x *= 3;

Similarly, a list assignment in list context produces the list of lvalues assigned to, and a list assignment in scalar context returns the number of elements produced by the expression on the right hand side of the assignment.

The three dotted bitwise assignment operators (&.= |.= ^.=) are new in Perl 5.22 and experimental. See "Bitwise String Operators".

Comma Operator

Binary "," is the comma operator. In scalar context it evaluates its left argument, throws that value away, then evaluates its right argument and returns that value. This is just like C's comma operator.

In list context, it's just the list argument separator, and inserts both its arguments into the list. These arguments are also evaluated from left to right.

The => operator (sometimes pronounced "fat comma") is a synonym for the comma except that it causes a word on its left to be interpreted as a string if it begins with a letter or underscore and is composed only of letters, digits and underscores. This includes operands that might otherwise be interpreted as operators, constants, single number v-strings or function calls. If in doubt about this behavior, the left operand can be quoted explicitly.

Otherwise, the => operator behaves exactly as the comma operator or list argument separator, according to context.

For example:

use constant FOO => "something";
my %h = ( FOO => 23 );

is equivalent to:

my %h = ("FOO", 23);

It is NOT:

my %h = ("something", 23);

The => operator is helpful in documenting the correspondence between keys and values in hashes, and other paired elements in lists.

%hash = ( $key => $value );
login( $username => $password );

The special quoting behavior ignores precedence, and hence may apply to part of the left operand:

print time.shift => "bbb";

That example prints something like "1314363215shiftbbb", because the => implicitly quotes the shift immediately on its left, ignoring the fact that time.shift is the entire left operand.

List Operators (Rightward)

On the right side of a list operator, the comma has very low precedence, such that it controls all comma-separated expressions found there. The only operators with lower precedence are the logical operators "and", "or", and "not", which may be used to evaluate calls to list operators without the need for parentheses:

open HANDLE, "< :utf8", "filename" or die "Can't open: $!\n";

However, some people find that code harder to read than writing it with parentheses:

open(HANDLE, "< :utf8", "filename") or die "Can't open: $!\n";

in which case you might as well just use the more customary "||" operator:

open(HANDLE, "< :utf8", "filename") || die "Can't open: $!\n";

See also discussion of list operators in "Terms and List Operators (Leftward)".

Logical Not

Unary "not" returns the logical negation of the expression to its right. It's the equivalent of "!" except for the very low precedence.

Logical And

Binary "and" returns the logical conjunction of the two surrounding expressions. It's equivalent to && except for the very low precedence. This means that it short-circuits: the right expression is evaluated only if the left expression is true.

Logical or and Exclusive Or

Binary "or" returns the logical disjunction of the two surrounding expressions. It's equivalent to || except for the very low precedence. This makes it useful for control flow:

print FH $data		or die "Can't write to FH: $!";

This means that it short-circuits: the right expression is evaluated only if the left expression is false. Due to its precedence, you must be careful to avoid using it as replacement for the || operator. It usually works out better for flow control than in assignments:

$x = $y or $z;              # bug: this is wrong
($x = $y) or $z;            # really means this
$x = $y || $z;              # better written this way

However, when it's a list-context assignment and you're trying to use || for control flow, you probably need "or" so that the assignment takes higher precedence.

@info = stat($file) || die;     # oops, scalar sense of stat!
@info = stat($file) or die;     # better, now @info gets its due

Then again, you could always use parentheses.

Binary "xor" returns the exclusive-OR of the two surrounding expressions. It cannot short-circuit (of course).

There is no low precedence operator for defined-OR.

C Operators Missing From Perl

Here is what C has that Perl doesn't:

Quote and Quote-like Operators

While we usually think of quotes as literal values, in Perl they function as operators, providing various kinds of interpolating and pattern matching capabilities. Perl provides customary quote characters for these behaviors, but also provides a way for you to choose your quote character for any of them. In the following table, a {} represents any pair of delimiters you choose.

Customary  Generic        Meaning	     Interpolates
	''	 q{}	      Literal		  no
	""	qq{}	      Literal		  yes
	``	qx{}	      Command		  yes*
		qw{}	     Word list		  no
	//	 m{}	   Pattern match	  yes*
		qr{}	      Pattern		  yes*
		 s{}{}	    Substitution	  yes*
		tr{}{}	  Transliteration	  no (but see below)
		 y{}{}	  Transliteration	  no (but see below)
    <<EOF                 here-doc            yes*
	* unless the delimiter is ''.

Non-bracketing delimiters use the same character fore and aft, but the four sorts of ASCII brackets (round, angle, square, curly) all nest, which means that

q{foo{bar}baz}

is the same as

'foo{bar}baz'

Note, however, that this does not always work for quoting Perl code:

$s = q{ if($x eq "}") ... }; # WRONG

is a syntax error. The Text::Balanced module (standard as of v5.8, and from CPAN before then) is able to do this properly.

There can be whitespace between the operator and the quoting characters, except when # is being used as the quoting character. q#foo# is parsed as the string foo, while pppppppppppppppppppppppppppppppppppppppppp is the operator q followed by a comment. Its argument will be taken from the next line. This allows you to write:

s {foo}  # Replace foo
  {bar}  # with bar.

The following escape sequences are available in constructs that interpolate, and in transliterations:

Sequence     Note  Description
\t                  tab               (HT, TAB)
\n                  newline           (NL)
\r                  return            (CR)
\f                  form feed         (FF)
\b                  backspace         (BS)
\a                  alarm (bell)      (BEL)
\e                  escape            (ESC)
\x{263A}     [1,8]  hex char          (example: SMILEY)
\x1b         [2,8]  restricted range hex char (example: ESC)
\N{name}     [3]    named Unicode character or character sequence
\N{U+263D}   [4,8]  Unicode character (example: FIRST QUARTER MOON)
\c[          [5]    control char      (example: chr(27))
\o{23072}    [6,8]  octal char        (example: SMILEY)
\033         [7,8]  restricted range octal char  (example: ESC)

NOTE: Unlike C and other languages, Perl has no \v escape sequence for the vertical tab (VT, which is 11 in both ASCII and EBCDIC), but you may use \N{VT}, \ck, \N{U+0b}, or \x0b. (\v does have meaning in regular expression patterns in Perl, see perlre.)

The following escape sequences are available in constructs that interpolate, but not in transliterations.

\l		lowercase next character only
\u		titlecase (not uppercase!) next character only
\L		lowercase all characters till \E or end of string
\U		uppercase all characters till \E or end of string
\F		foldcase all characters till \E or end of string
\Q          quote (disable) pattern metacharacters till \E or
            end of string
\E		end either case modification or quoted section
		(whichever was last seen)

See "quotemeta" in perlfunc for the exact definition of characters that are quoted by \Q.

\L, \U, \F, and \Q can stack, in which case you need one \E for each. For example:

say"This \Qquoting \ubusiness \Uhere isn't quite\E done yet,\E is it?";
This quoting\ Business\ HERE\ ISN\'T\ QUITE\ done\ yet\, is it?

If a pppppppppppppppppppppppppppppppppppppppppp form that includes LC_CTYPE is in effect (see perllocale), the case map used by \l, \L, \u, and \U is taken from the current locale. If Unicode (for example, \N{} or code points of 0x100 or beyond) is being used, the case map used by \l, \L, \u, and \U is as defined by Unicode. That means that case-mapping a single character can sometimes produce a sequence of several characters. Under pppppppppppppppppppppppppppppppppppppppppp, \F produces the same results as \L for all locales but a UTF-8 one, where it instead uses the Unicode definition.

All systems use the virtual "\n" to represent a line terminator, called a "newline". There is no such thing as an unvarying, physical newline character. It is only an illusion that the operating system, device drivers, C libraries, and Perl all conspire to preserve. Not all systems read "\r" as ASCII CR and "\n" as ASCII LF. For example, on the ancient Macs (pre-MacOS X) of yesteryear, these used to be reversed, and on systems without a line terminator, printing "\n" might emit no actual data. In general, use "\n" when you mean a "newline" for your system, but use the literal ASCII when you need an exact character. For example, most networking protocols expect and prefer a CR+LF ("\015\012" or "\cM\cJ") for line terminators, and although they often accept just "\012", they seldom tolerate just "\015". If you get in the habit of using "\n" for networking, you may be burned some day.

For constructs that do interpolate, variables beginning with "$" or "@" are interpolated. Subscripted variables such as $a[3] or $href->{key}[0] are also interpolated, as are array and hash slices. But method calls such as $obj->meth are not.

Interpolating an array or slice interpolates the elements in order, separated by the value of $", so is equivalent to interpolating pppppppppppppppppppppppppppppppppppppppppp. "Punctuation" arrays such as @* are usually interpolated only if the name is enclosed in braces @{*}, but the arrays @_, @+, and @- are interpolated even without braces.

For double-quoted strings, the quoting from \Q is applied after interpolation and escapes are processed.

"abc\Qfoo\tbar$s\Exyz"

is equivalent to

"abc" . quotemeta("foo\tbar$s") . "xyz"

For the pattern of regex operators (qr//, m// and s///), the quoting from \Q is applied after interpolation is processed, but before escapes are processed. This allows the pattern to match literally (except for $ and @). For example, the following matches:

'\s\t' =~ /\Q\s\t/

Because $ or @ trigger interpolation, you'll need to use something like /\Quser\E\@\Qhost/ to match them literally.

Patterns are subject to an additional level of interpretation as a regular expression. This is done as a second pass, after variables are interpolated, so that regular expressions may be incorporated into the pattern from the variables. If this is not what you want, use \Q to interpolate a variable literally.

Apart from the behavior described above, Perl does not expand multiple levels of interpolation. In particular, contrary to the expectations of shell programmers, back-quotes do NOT interpolate within double quotes, nor do single quotes impede evaluation of variables when used within double quotes.

Regexp Quote-Like Operators

Here are the quote-like operators that apply to pattern matching and related activities.

Quote-Like Operators

Gory details of parsing quoted constructs

When presented with something that might have several different interpretations, Perl uses the DWIM (that's "Do What I Mean") principle to pick the most probable interpretation. This strategy is so successful that Perl programmers often do not suspect the ambivalence of what they write. But from time to time, Perl's notions differ substantially from what the author honestly meant.

This section hopes to clarify how Perl handles quoted constructs. Although the most common reason to learn this is to unravel labyrinthine regular expressions, because the initial steps of parsing are the same for all quoting operators, they are all discussed together.

The most important Perl parsing rule is the first one discussed below: when processing a quoted construct, Perl first finds the end of that construct, then interprets its contents. If you understand this rule, you may skip the rest of this section on the first reading. The other rules are likely to contradict the user's expectations much less frequently than this first one.

Some passes discussed below are performed concurrently, but because their results are the same, we consider them individually. For different quoting constructs, Perl performs different numbers of passes, from one to four, but these passes are always performed in the same order.

I/O Operators

There are several I/O operators you should know about.

A string enclosed by backticks (grave accents) first undergoes double-quote interpolation. It is then interpreted as an external command, and the output of that command is the value of the backtick string, like in a shell. In scalar context, a single string consisting of all output is returned. In list context, a list of values is returned, one per line of output. (You can set $/ to use a different line terminator.) The command is executed each time the pseudo-literal is evaluated. The status value of the command is returned in $? (see perlvar for the interpretation of $?). Unlike in csh, no translation is done on the return data--newlines remain newlines. Unlike in any of the shells, single quotes do not hide variable names in the command from interpretation. To pass a literal dollar-sign through to the shell you need to hide it with a backslash. The generalized form of backticks is qx//. (Because backticks always undergo shell expansion as well, see perlsec for security concerns.)

In scalar context, evaluating a filehandle in angle brackets yields the next line from that file (the newline, if any, included), or undef at end-of-file or on error. When $/ is set to undef (sometimes known as file-slurp mode) and the file is empty, it returns '' the first time, followed by undef subsequently.

Ordinarily you must assign the returned value to a variable, but there is one situation where an automatic assignment happens. If and only if the input symbol is the only thing inside the conditional of a while statement (even if disguised as a for(;;) loop), the value is automatically assigned to the global variable $_, destroying whatever was there previously. (This may seem like an odd thing to you, but you'll use the construct in almost every Perl script you write.) The $_ variable is not implicitly localized. You'll have to put a pppppppppppppppppppppppppppppppppppppppppp before the loop if you want that to happen.

The following lines are equivalent:

while (defined($_ = <STDIN>)) { print; }
while ($_ = <STDIN>) { print; }
while (<STDIN>) { print; }
for (;<STDIN>;) { print; }
print while defined($_ = <STDIN>);
print while ($_ = <STDIN>);
print while <STDIN>;

This also behaves similarly, but assigns to a lexical variable instead of to $_:

while (my $line = <STDIN>) { print $line }

In these loop constructs, the assigned value (whether assignment is automatic or explicit) is then tested to see whether it is defined. The defined test avoids problems where the line has a string value that would be treated as false by Perl; for example a "" or a "0" with no trailing newline. If you really mean for such values to terminate the loop, they should be tested for explicitly:

while (($_ = <STDIN>) ne '0') { ... }
while (<STDIN>) { last unless $_; ... }

In other boolean contexts, <FILEHANDLE> without an explicit defined test or comparison elicits a warning if the pppppppppppppppppppppppppppppppppppppppppp pragma or the -w command-line switch (the $^W variable) is in effect.

The filehandles STDIN, STDOUT, and STDERR are predefined. (The filehandles stdin, stdout, and stderr will also work except in packages, where they would be interpreted as local identifiers rather than global.) Additional filehandles may be created with the open() function, amongst others. See perlopentut and "open" in perlfunc for details on this.

If a <FILEHANDLE> is used in a context that is looking for a list, a list comprising all input lines is returned, one line per list element. It's easy to grow to a rather large data space this way, so use with care.

<FILEHANDLE> may also be spelled readline(*FILEHANDLE). See "readline" in perlfunc.

The null filehandle <> is special: it can be used to emulate the behavior of sed and awk, and any other Unix filter program that takes a list of filenames, doing the same to each line of input from all of them. Input from <> comes either from standard input, or from each file listed on the command line. Here's how it works: the first time <> is evaluated, the @ARGV array is checked, and if it is empty, $ARGV[0] is set to "-", which when opened gives you standard input. The @ARGV array is then processed as a list of filenames. The loop

while (<>) {
	...			# code for each line
}

is equivalent to the following Perl-like pseudo code:

unshift(@ARGV, '-') unless @ARGV;
while ($ARGV = shift) {
	open(ARGV, $ARGV);
	while (<ARGV>) {
	    ...		# code for each line
	}
}

except that it isn't so cumbersome to say, and will actually work. It really does shift the @ARGV array and put the current filename into the $ARGV variable. It also uses filehandle ARGV internally. <> is just a synonym for <ARGV>, which is magical. (The pseudo code above doesn't work because it treats <ARGV> as non-magical.)

Since the null filehandle uses the two argument form of "open" in perlfunc it interprets special characters, so if you have a script like this:

while (<>) {
    print;
}

and call it with pppppppppppppppppppppppppppppppppppppppppp, it actually opens a pipe, executes the rm command and reads rm's output from that pipe. If you want all items in @ARGV to be interpreted as file names, you can use the module ARGV::readonly from CPAN, or use the double bracket:

while (<<>>) {
    print;
}

Using double angle brackets inside of a while causes the open to use the three argument form (with the second argument being <), so all arguments in ARGV are treated as literal filenames (including "-"). (Note that for convenience, if you use <<>> and if @ARGV is empty, it will still read from the standard input.)

You can modify @ARGV before the first <> as long as the array ends up containing the list of filenames you really want. Line numbers ($.) continue as though the input were one big happy file. See the example in "eof" in perlfunc for how to reset line numbers on each file.

If you want to set @ARGV to your own list of files, go right ahead. This sets @ARGV to all plain text files if no @ARGV was given:

@ARGV = grep { -f && -T } glob('*') unless @ARGV;

You can even set them to pipe commands. For example, this automatically filters compressed arguments through gzip:

@ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;

If you want to pass switches into your script, you can use one of the Getopts modules or put a loop on the front like this:

while ($_ = $ARGV[0], /^-/) {
	shift;
    last if /^--$/;
	if (/^-D(.*)/) { $debug = $1 }
	if (/^-v/)     { $verbose++  }
	# ...		# other switches
}
while (<>) {
	# ...		# code for each line
}

The <> symbol will return undef for end-of-file only once. If you call it again after this, it will assume you are processing another @ARGV list, and if you haven't set @ARGV, will read input from STDIN.

If what the angle brackets contain is a simple scalar variable (for example, $foo), then that variable contains the name of the filehandle to input from, or its typeglob, or a reference to the same. For example:

$fh = \*STDIN;
$line = <$fh>;

If what's within the angle brackets is neither a filehandle nor a simple scalar variable containing a filehandle name, typeglob, or typeglob reference, it is interpreted as a filename pattern to be globbed, and either a list of filenames or the next filename in the list is returned, depending on context. This distinction is determined on syntactic grounds alone. That means <$x> is always a readline() from an indirect handle, but <$hash{key}> is always a glob(). That's because $x is a simple scalar variable, but $hash{key} is not--it's a hash element. Even <$x > (note the extra space) is treated as glob("$x "), not readline($x).

One level of double-quote interpretation is done first, but you can't say <$foo> because that's an indirect filehandle as explained in the previous paragraph. (In older versions of Perl, programmers would insert curly brackets to force interpretation as a filename glob: <${foo}>. These days, it's considered cleaner to call the internal function directly as glob($foo), which is probably the right way to have done it in the first place.) For example:

while (<*.c>) {
	chmod 0644, $_;
}

is roughly equivalent to:

open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
while (<FOO>) {
	chomp;
	chmod 0644, $_;
}

except that the globbing is actually done internally using the standard File::Glob extension. Of course, the shortest way to do the above is:

chmod 0644, <*.c>;

A (file)glob evaluates its (embedded) argument only when it is starting a new list. All values must be read before it will start over. In list context, this isn't important because you automatically get them all anyway. However, in scalar context the operator returns the next value each time it's called, or undef when the list has run out. As with filehandle reads, an automatic defined is generated when the glob occurs in the test part of a while, because legal glob returns (for example, a file called 0) would otherwise terminate the loop. Again, undef is returned only once. So if you're expecting a single value from a glob, it is much better to say

($file) = <blurch*>;

than

$file = <blurch*>;

because the latter will alternate between returning a filename and returning false.

If you're trying to do variable interpolation, it's definitely better to use the glob() function, because the older notation can cause people to become confused with the indirect filehandle notation.

@files = glob("$dir/*.[ch]");
@files = glob($files[$i]);

Constant Folding

Like C, Perl does a certain amount of expression evaluation at compile time whenever it determines that all arguments to an operator are static and have no side effects. In particular, string concatenation happens at compile time between literals that don't do variable substitution. Backslash interpolation also happens at compile time. You can say

  'Now is the time for all'
. "\n" 
.  'good men to come to.'

and this all reduces to one string internally. Likewise, if you say

foreach $file (@filenames) {
	if (-s $file > 5 + 100 * 2**16) {  }
}

the compiler precomputes the number which that expression represents so that the interpreter won't have to.

No-ops

Perl doesn't officially have a no-op operator, but the bare constants 0 and 1 are special-cased not to produce a warning in void context, so you can for example safely do

1 while foo();

Bitwise String Operators

Bitstrings of any size may be manipulated by the bitwise operators (~ | & ^).

If the operands to a binary bitwise op are strings of different sizes, | and ^ ops act as though the shorter operand had additional zero bits on the right, while the & op acts as though the longer operand were truncated to the length of the shorter. The granularity for such extension or truncation is one or more bytes.

# ASCII-based examples
print "j p \n" ^ " a h";        	# prints "JAPH\n"
print "JA" | "  ph\n";          	# prints "japh\n"
print "japh\nJunk" & '_____';   	# prints "JAPH\n";
print 'p N$' ^ " E<H\n";		# prints "Perl\n";

If you are intending to manipulate bitstrings, be certain that you're supplying bitstrings: If an operand is a number, that will imply a numeric bitwise operation. You may explicitly show which type of operation you intend by using "" or 0+, as in the examples below.

$foo =  150  |  105;	# yields 255  (0x96 | 0x69 is 0xFF)
$foo = '150' |  105;	# yields 255
$foo =  150  | '105';	# yields 255
$foo = '150' | '105';	# yields string '155' (under ASCII)
$baz = 0+$foo & 0+$bar;	# both ops explicitly numeric
$biz = "$foo" ^ "$bar";	# both ops explicitly stringy

This somewhat unpredictable behavior can be avoided with the experimental "bitwise" feature, new in Perl 5.22. You can enable it via pppppppppppppppppppppppppppppppppppppppppp. By default, it will warn unless the "experimental::bitwise" warnings category has been disabled. (pppppppppppppppppppppppppppppppppppppppppp will enable the feature and disable the warning.) Under this feature, the four standard bitwise operators (~ | & ^) are always numeric. Adding a dot after each operator (~. |. &. ^.) forces it to treat its operands as strings:

use experimental "bitwise";
$foo =  150  |  105;	# yields 255  (0x96 | 0x69 is 0xFF)
$foo = '150' |  105;	# yields 255
$foo =  150  | '105';	# yields 255
$foo = '150' | '105';	# yields 255
$foo =  150  |. 105;	# yields string '155'
$foo = '150' |. 105;	# yields string '155'
$foo =  150  |.'105';	# yields string '155'
$foo = '150' |.'105';	# yields string '155'
$baz = $foo &  $bar;	# both operands numeric
$biz = $foo ^. $bar;	# both operands stringy

The assignment variants of these operators (&= |= ^= &.= |.= ^.=) behave likewise under the feature.

The behavior of these operators is problematic (and subject to change) if either or both of the strings are encoded in UTF-8 (see "Byte and Character Semantics" in perlunicode.

See "vec" in perlfunc for information on how to manipulate individual bits in a bit vector.

Integer Arithmetic

By default, Perl assumes that it must do most of its arithmetic in floating point. But by saying

use integer;

you may tell the compiler to use integer operations (see integer for a detailed explanation) from here to the end of the enclosing BLOCK. An inner BLOCK may countermand this by saying

no integer;

which lasts until the end of that BLOCK. Note that this doesn't mean everything is an integer, merely that Perl will use integer operations for arithmetic, comparison, and bitwise operators. For example, even under pppppppppppppppppppppppppppppppppppppppppp, if you take the sqrt(2), you'll still get 1.4142135623731 or so.

Used on numbers, the bitwise operators (& | ^ ~ << >>) always produce integral results. (But see also "Bitwise String Operators".) However, pppppppppppppppppppppppppppppppppppppppppp still has meaning for them. By default, their results are interpreted as unsigned integers, but if pppppppppppppppppppppppppppppppppppppppppp is in effect, their results are interpreted as signed integers. For example, ~0 usually evaluates to a large integral value. However, pppppppppppppppppppppppppppppppppppppppppp is -1 on two's-complement machines.

Floating-point Arithmetic

While pppppppppppppppppppppppppppppppppppppppppp provides integer-only arithmetic, there is no analogous mechanism to provide automatic rounding or truncation to a certain number of decimal places. For rounding to a certain number of digits, sprintf() or printf() is usually the easiest route. See perlfaq4.

Floating-point numbers are only approximations to what a mathematician would call real numbers. There are infinitely more reals than floats, so some corners must be cut. For example:

printf "%.20g\n", 123456789123456789;
#        produces 123456789123456784

Testing for exact floating-point equality or inequality is not a good idea. Here's a (relatively expensive) work-around to compare whether two floating-point numbers are equal to a particular number of decimal places. See Knuth, volume II, for a more robust treatment of this topic.

sub fp_equal {
	my ($X, $Y, $POINTS) = @_;
	my ($tX, $tY);
	$tX = sprintf("%.${POINTS}g", $X);
	$tY = sprintf("%.${POINTS}g", $Y);
	return $tX eq $tY;
}

The POSIX module (part of the standard perl distribution) implements ceil(), floor(), and other mathematical and trigonometric functions. The Math::Complex module (part of the standard perl distribution) defines mathematical functions that work on both the reals and the imaginary numbers. Math::Complex is not as efficient as POSIX, but POSIX can't work with complex numbers.

Rounding in financial applications can have serious implications, and the rounding method used should be specified precisely. In these cases, it probably pays not to trust whichever system rounding is being used by Perl, but to instead implement the rounding function you need yourself.

Bigger Numbers

The standard Math::BigInt, Math::BigRat, and Math::BigFloat modules, along with the bignum, bigint, and bigrat pragmas, provide variable-precision arithmetic and overloaded operators, although they're currently pretty slow. At the cost of some space and considerable speed, they avoid the normal pitfalls associated with limited-precision representations.

	use 5.010;
	use bigint;  # easy interface to Math::BigInt
	$x = 123456789123456789;
	say $x * $x;
+15241578780673678515622620750190521

Or with rationals:

use 5.010;
use bigrat;
$x = 3/22;
$y = 4/6;
say "x/y is ", $x/$y;
say "x*y is ", $x*$y;
x/y is 9/44
x*y is 1/11

Several modules let you calculate with unlimited or fixed precision (bound only by memory and CPU time). There are also some non-standard modules that provide faster implementations via external C libraries.

Here is a short, but incomplete summary:

Math::String           treat string sequences like numbers
Math::FixedPrecision   calculate with a fixed precision
Math::Currency         for currency calculations
Bit::Vector            manipulate bit vectors fast (uses C)
Math::BigIntFast       Bit::Vector wrapper for big numbers
Math::Pari             provides access to the Pari C library
Math::Cephes           uses the external Cephes C library (no
                       big numbers)
Math::Cephes::Fraction fractions via the Cephes library
Math::GMP              another one using an external C library
Math::GMPz             an alternative interface to libgmp's big ints
Math::GMPq             an interface to libgmp's fraction numbers
Math::GMPf             an interface to libgmp's floating point numbers

Choose wisely.