wait_one_child is an undocumented method of P:FM, but it seems like that would be useful in your case. An attempt at psueudo-code:

while ($stuff_to_do) {
  for my $job (@job_list) {
    $pm->start($job) and next;
    $pm->finish();
  }
  if ($still_doing_stuff) {
    # run_on_finish might push more stuff to @job_list
    $pm->wait_one_child();
  }
}
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