# Record fd3 for later use
open($orig_fd{3},">&=",3);
for $id (1..10) {
  open($child{$id,3},">",$tmpfile{$i});
  # make $child{$id,3} file descriptor 3 for me and all future children
  connect_filedescriptor(3,$child{$id,3});
  # start the child
  open3(...)
}

sub reaper {
  # When a child dies
  seek $child{$id,3},0,0;
  # Print the captured fd3 to the parent's original fd3
  print $orig_fd{3} <$child{$id,3}>;
  close $child{$id,3};
}