=over =item $ARG =item $_ X<$_> X<$ARG> The default input and pattern-searching space. The following pairs are equivalent: while (<>) {...} # equivalent only in while! while (defined($_ = <>)) {...} /^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: =over 3 =item * The following functions: abs, alarm, chomp, chop, chr, chroot, cos, defined, eval, exp, glob, hex, int, lc, lcfirst, length, log, lstat, mkdir, oct, ord, pos, print, quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only), rmdir, sin, split (on its second argument), sqrt, stat, study, uc, ucfirst, unlink, unpack. =item * All file tests (C<-f>, C<-d>) except for C<-t>, which defaults to STDIN. See L =item * The pattern matching operations C, C and C (aka C) when used without an C<=~> operator. =item * The default iterator variable in a C loop if no other variable is supplied. =item * The implicit iterator variable in the grep() and map() functions. =item * The implicit variable of given(). =item * The default place to put an input record when a C<< >> operation's result is tested by itself as the sole criterion of a C test. Outside a C test, this will not happen. =back (Mnemonic: underline is understood in certain operations.) =back