#$ARG

#$_

The default input and pattern-searching space. The following pairs are equivalent:

while (<>) {...}	# equivalent only in while!
while (defined($_ = <>)) {...}

/^Subject:/
$_ =~ /^Subject:/

tr/a-z/A-Z/
$_ =~ tr/a-z/A-Z/

chomp
chomp($_)

Here are the places where Perl will assume $_ even if you don't use it:

• Various unary functions, including functions like ord() and int(), as well as the all file tests (-f, -d) except for -t, which defaults to STDIN.

• Various list functions like print() and unlink().

• The pattern matching operations m//, s///, and tr/// when used without an =~ operator.

• The default iterator variable in a foreach loop if no other variable is supplied.

• The implicit iterator variable in the grep() and map() functions.

• The default place to put an input record when a <FH> operation's result is tested by itself as the sole criterion of a while test. Outside a while test, this will not happen.

As $_ is a global variable, this may lead in some cases to unwanted side-effects. As of perl 5.9.1, you can now use a lexical version of $_ by declaring it in a file or in a block with my. Moreover, declaring our $_ restores the global $_ in the current scope.

(Mnemonic: underline is understood in certain operations.)

Perldoc Browser is maintained by Dan Book (DBOOK). Please contact him via the GitHub issue tracker or email regarding any issues with the site itself, search, or rendering of documentation.

The Perl documentation is maintained by the Perl 5 Porters in the development of Perl. Please contact them via the Perl issue tracker, the mailing list, or IRC to report any issues with the contents or format of the documentation.