Perl5 リファレンス

       fileno FILEHANDLE
               Returns the file descriptor for a filehandle, or
               undefined if the filehandle is not open.  This is
               mainly useful for constructing bitmaps for
               `select' and low-level POSIX tty-handling
               operations.  If FILEHANDLE is an expression, the
               value is taken as an indirect filehandle,
               generally its name.

               You can use this to find out whether two handles
               refer to the same underlying descriptor:

                   if (fileno(THIS) == fileno(THAT)) {
                       print "THIS and THAT are dups\n";