my $old_work = get_initial_work(); # hash reference
my $new_work = {};
while (%$old_work) {
    for my $item (keys %$old_work) {
        my $job = delete $old_work->{$item};
        for (new_work($job)) {
            $new_work{$item . $_} = $_;
        }
    }
    ($new_work, $old_work) = ({}, $new_work);
}