just wait for the CHILD in the parent:

my ($i, $return, $pid, @pids);
   for ($i = 1; $i <= 3; $i++) {
       if ($pid=fork) {
           push @pids, $pid;
           waitpid($pid,0);
       } elsif (defined $pid) {
           $return = &my_function($i);
           print "Returned: $return\n";
           exit;
       }
   }
print "All done.\n";
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