You can pass back whatever you want. I just started using this module recently, and needed to pass back a delete list and an insert list. So my run_on_finish sub starts off with this ..

   $pfm->run_on_finish (
      sub {

        my ($pid, $exit_code, $ident, $exit_signal, $core_dump,
          $data_structure_reference) = @_;
        if ( defined $data_structure_reference ) {

          my $delete_list = $data_structure_reference->{delete_list};
          my $insert_list = $data_structure_reference->{insert_list};
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That sets things up for when a kid finishes and puts something into the return parameter. Then, in the kid code, I do this:

            if ( keys %delete_list || keys %insert_list ) {

              $pfm->finish ( 0,
                { delete_list => \%delete_list,
                  insert_list => \%insert_list } );

            } else {

              $pfm->finish;
            }
[download]

and it just works. Magic.