my $pid = system(1, $^X, -e => 'sleep 30 while 1')
    or die("system,1: $!\n");

kill(9, $pid)
    or die("kill: $!\n");

if (kill(0, $pid)) {
   print("Process $pid still exists\n") ;
} else {
   print("kill 0: $!\n") ;
}

waitpid($pid, 0)
    or die("waitpid: $!\n");

if (kill(0, $pid)) {
   print("Process $pid still exists\n") ;
} else {
   print("kill 0: $!\n") ;
}

##</code><code>##

Process 4912 still exists
kill 0: Invalid argument