from perlop

       The null filehandle <> is special: it can be used to emulate the
       behavior of sed and awk.  Input from <> comes either from standard
       input, or from each file listed on the command line.  Here’s how it
       works: the first time <> is evaluated, the @ARGV array is checked, and
       if it is empty, $ARGV[0] is set to "-", which when opened gives you
       standard input.  The @ARGV array is then processed as a list of
       filenames.  The loop


           while (<>) {
               ...                     # code for each line
           }


       is equivalent to the following Perl-like pseudo code:


           unshift(@ARGV, '-') unless @ARGV;
           while ($ARGV = shift) {
               open(ARGV, $ARGV);
               while (<ARGV>) {
                   ...         # code for each line
               }
           }