Try isolating the "must fork" code; trying to get the redo sorted out was driving me nuts:

sub insistent_fork {
  my $pid = undef;
  FORK: {
    if (defined ($pid = fork())) {
      return $pid;
    }
    elsif ($! =~ /No more process/) {
      sleep 5;
      redo FORK;
    }
    else {
      die "Can't fork: $!\n";
    }
  }
}
[download]

You can pair this with the original code I posted, substituting insistent_fork for fork. (Works on OS X.)