=over =item $ARG =item $_ The default input and pattern-searching space. The following pairs are equivalent: while (<>) {...} # equivalent only in while! while (defined($_ = <>)) {...} /^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 * Various unary functions, including functions like ord() and int(), as well as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to STDIN. =item * Various list functions like print() and unlink(). =item * The pattern matching operations C, C, and 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 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