use Proc::Queue size => 1,
                ignore_childs => 1,
                qw(running_now);

someLoop {
  ...
  ... do stuff ...
  ...
  if(running_now < 1) {
    my $pid = fork;
    defined $pid or die ...
    unless ($pid) {
       ... do stuff in child ...
       exit(0);
    }
  }
  ...
  ... carry on doing stuff while child runs ...
  ...
}
[download]