I think I know what you are asking, but I'm not sure.

You want to, for an example, write a spell check program, in which you want each letter to be a child process:

##Extremely simple code
for $x ('A'..'Z') {
 if ($pid=fork) {
   ## This child checks the letter $x
   exit;
 }
}
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The problem is that you want to "simultaneously execute the subroutine" AND you want the children to return their results in the exact order they were called. There is no simple way to have both: since each child is doing a different task, you have no way of knowing how soon each will finish. You can have the parent wait for the a child to finish before starting the next child, but that's the same as a non-forked loop.

If the return order does not matter, you can just gather the children's results as they finish. (How exactly you do this depends on a few things: appending to a temporary file may be the easiest way.)

If the order DOES matter, you must have a way for the parent to identify the children. In the example above, the child could output it's "letter" as the first character of it's result. Then it is up to the parent to sort out the results and display them in a pretty manner.

Here's some quick code illustrating some of the above:

$tmpfile = "/tmp/results.$^T$$";
for $x ('A'..'Z') {
  if (fork) { ## We don't need to know PID in this example
    $results = &SpellCheck($x);
    if (open (TMPFILE, ">> $tmpfile")) {
      print TMPFILE "$x|$results\n";
      close(TMPFILE);
    }
    exit; ## Done with this child
  }
}
## Wait for all the children to finish, then...

if (open (TMPFILE, $tmpfile)) {
  @results = <TMPFILE>;
  close(RESULTS);
  sort(@results); ## Works well for this example!!
}
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In the example above, @results will contain the results from all 26 children, sorted in alphabetic order.