$_

$_

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:

• The following functions use $_ as a default argument:

  abs, alarm, chomp, chop, chr, chroot, cos, defined, eval, evalbytes, exp, fc, glob, hex, int, lc, lcfirst, length, log, lstat, mkdir, oct, ord, pos, print, printf, quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only), rmdir, say, sin, split (for its second argument), sqrt, stat, study, uc, ucfirst, unlink, unpack.

• All file tests (-f , -d ) except for -t , which defaults to STDIN. See -X

• The pattern matching operations m//, s/// and tr/// (aka y///) 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 implicit variable of given() .

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

$_ is by default a global variable. However, as of perl v5.10.0, you can 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. Though this seemed like a good idea at the time it was introduced, lexical $_ actually causes more problems than it solves. If you call a function that expects to be passed information via $_ , it may or may not work, depending on how the function is written, there not being any easy way to solve this. Just avoid lexical $_ , unless you are feeling particularly masochistic. For this reason lexical $_ is still experimental and will produce a warning unless warnings have been disabled. As with other experimental features, the behavior of lexical $_ is subject to change without notice, including change into a fatal error.

Mnemonic: underline is understood in certain operations.