You are viewing the version of this documentation from Perl 5.40.0-RC1. This is a development version of Perl.

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

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

$_ =~ /^Subject:/

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


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" in perlfunc

  • 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 a global variable.

However, between perl v5.10.0 and v5.24.0, it could be used lexically by writing my $_. Making $_ refer to the global $_ in the same scope was then possible with our $_. This experimental feature was removed and is now a fatal error, but you may encounter it in older code.

Mnemonic: underline is understood in certain operations.