use IO::Select qw( );

my $sel = IO::Select->new(\*STDIN, \*TNIN);

MAIN_LOOP:
while (my @ready = $sel->can_read()) {
   for my $fh (@ready) {
      my $bytes_read = sysread($fh, my $data='', 4096)
         or last MAIN_LOOP;

      if ($fh == \*STDIN) {
         print TNOUT do_this($data);
      } else {
         print STDOUT do_that($data);
      }
   }
}