Windows - I'm not sure there are any others.

Unix - ctrl-z produces something - SIGSTOP, I think. And "kill -1 $perl_pid" (from another shell) produces SIGHUP (a popular way of alerting daemons to re-read their config file), "kill -2 $perl_pid" (from another shell) produces SIGINT (same as ctrl-c), "kill -3 $perl_pid" ... etc. Anything else requires you to poll the keyboard for special keys, such as using Term::ReadKey:

  use Term::ReadKey;
  ReadMode 4;
  if (my $key = ReadKey(-1) and $key = ' ')
  {
    # space was pressed
    close(INPUT_HANDLE);
  }
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Just put something like that at various places in your loop, although if you're currently blocked on something else, this won't really help.