sub hash_filter {
    my $source = shift;
    my $filter = shift;

    return {
        map {
            ref $filter->{$_} eq 'HASH'
            ? ref $source->{$_} eq 'HASH'
              ? ($_, hash_filter( $source->{$_}, $filter->{$_} ))
              : croak "bad filter: on '$_', expected HASH\n"
            : ($_, $source->{$_})
        } grep {
            exists $source->{$_}
        } keys %$filter
    }
}

##</code><code>##

sub hash_filter {
    return {
        map {
            ref $_[1]->{$_} eq 'HASH'
            ? ref $_[0]->{$_} eq 'HASH'
              ? ($_, hash_filter( $_[0]->{$_}, $_[1]->{$_} ))
              : croak "bad filter: on '$_', expected HASH\n"
            : ($_, $_[0]->{$_})
        } grep { exists $_[0]->{$_} } keys %{$_[1]}
    }
}