I don't think waitpid() is useful here since I don't know the order which the jobs will finish

Usually you don't need to reap the children in the same order they finish. Something like...

for my $pid (@pids) {
  my $r = waitpid($pid, 0);
  # do error checking here, and ensure that $r is really $pid
}
[download]

... will do.

The only drawback of this approach is that if you run ps on the machine you will see some zombie processes in some cases. It is fully harmless but some people gets nervous when they see zombies!