How about this?

END{
 $ENV{"PARENT"} = $$;
 if ($pid = fork()) {
   //do whatever your script does to end
 else {
   wait({$ENV{"PARENT"}});
   exec (foo);
 }
}
[download]

Although considering how fork works, you could just do

END {
 if ($pid = fork()) {
  wait($pid);
  exec(foo);
 }
 else {
  //finish your process off
 }
}
[download]

Now, either way, the process that wait's is still going to have a copy of all the descriptors and stuff, but that shouldn't be a huge problem.