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

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

                  /^Subject:/
                  $_ =~ /^Subject:/

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

                  chop
                  chop($_)

             (Mnemonic: underline is understood in certain opera-
             tions.)