sub compare_hashtrees {
    my ($tree_1, $tree_2) = @_;

    my @tree_1_only;

    foreach my $tree_1_key (keys %$tree_1) {
        my $sub_arrays_not_in_tree2 = [];
        if ( ! exists $tree_2->{$tree_1_key} ) {
            # the subtree is missing from tree_2 - expand it
            $sub_arrays_not_in_tree2
                 = expand_tree_to_arrays($tree_1->{$tree_1_key});
        }
        elsif ( ref $tree_1->{$tree_1_key} ) {
            # another level of hashing
            $sub_arrays_not_in_tree2
                    = compare_hashtrees($tree_1->{$tree_1_key}, $tree_2->{$tree_1_key});
        }
        foreach my $sub_array (@$sub_arrays_not_in_tree2) {
            push @tree_1_only, [$tree_1_key, @$sub_array];
        }
    }
    return \@tree_1_only;
}

sub expand_tree_to_arrays {
    my ($tree) = @_;

    if ( ref $tree ) {
        my @expanded;
        foreach my $key (sort keys %$tree) {
            my $sub_expanded = expand_tree_to_arrays($tree->{$key});
            foreach my $path (@$sub_expanded) {
                push @expanded, [$key, @$path];
            }
        }
        return \@expanded;
    }
    else {
#        return [ [$tree] ]; # its a scalar
         return [ [ ] ];
    }
}