`runN`

Here's the main logic of the program and it is awesome:

        my %pid;
        while (@ARGV) {
          if (keys(%pid) < $opt{n}) {
            $pid{spawn(@cmd, shift @ARGV)} = 1;
          } else {
            delete $pid{wait()};
          }
        }

        1 while wait() >= 0;

I glow with pride every time I see delete $pid{wait()}

spawn merely forks a subprocess to run the command, returns the pid