my @slot_in_use = (0, 0);
my @slot_names = qw/one two/;

sub find_slot () {
  for (my $i = 0; $i < @slot_in_use; $i++)
    { unless ($slot_in_use[$i]) { $slot_in_use[$i] = 1; return $i } }
  die "slot table full";
}

$pm->run_on_finish(sub {
  my ($c_pid, $c_exit, $c_slot) = @_;
  $slot_in_use[$c_slot] = 0; # release slot
});

foreach my $n (@data) {
  my $slot = find_slot;
  $pm->start($slot) and next;
  # ...
  $pm->finish;
}