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.)