my %results = ();
$pfm->run_on_finish( sub {
    my ($pid, $exit_code, $ident, $exit_signal, $core_dump, $data_structure_reference) = @_;
    my $data = Sereal::Decoder::decode_sereal($$data_structure_reference);
    # surely this is sequential code here so no need to lock %results, right?
    @results{keys %$data} = values %$data;
});