The solution I can immediately think of is to create the activity bar (the 'monitoring process') first, and have it create the other as the child so that it knows its pid and can establish ipc structures before/after the fork. Something like the code below could be used.

pipe my $read, my $write;
if (my $pid = fork()) {
  # monitor process
  close $write;
  read $read and update progress
} else {
  close $read;
  while (1) {
    do stuff;
    print $write "status";
  }
}
[download]

This code does only creates the one other process however, so if you really want two subprocesses created you have to wrap it in another fork.

--
integral, resident of freenode's #perl