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

  my %output;

  foreach ( keys %$filter ) {
    if ( exists $source->{$_} ) {

      if ( ref $filter->{$_} eq 'HASH' ) {
        croak "bad filter: on '$_', expected HASH\n"
          unless ( ref $source->{$_} eq 'HASH' );

        $output{$_} = hash_filter( $source->{$_}, $filter->{$_} );
      }
      else {
        $output{$_} = $source->{$_};
      }

    }
  }

  return \%output;
}