$_      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:

            *  Various unary functions, including functions like ord() and
               int(), as well as the all file tests ("-f", "-d") except for
               "-t", which defaults to STDIN.

            *  Various list functions like print() and unlink().

            *  The pattern matching operations "m//", "s///", and "tr///"
               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 default place to put an input record when a "<FH>"
               operation's result is tested by itself as the sole criterion
               of a "while" test. Outside a "while" test, this will not
               happen.

            (Mnemonic: underline is understood in certain operations.)