my $source = {
    f1 => 'garbage',
    f2 => 'more garbage',
    f3 => 'important data',
    f4 => {
        this => 'sub hash',
        is   => 'garbage'
    },
    f5 => {
        f6 => 'more important data',
        f7 => {
            more => 'garbage',
            f8   => 'important data',
        },
        f9 => 'garbage',
    },
    f10 => [ 'important', 'data' ],
    f11 => [ 'more', 'garbage' ]
};

##</code><code>##

my $filter = {
    f3 => 1,
    f5 => {
        f6 => 1,
        f7 => {
            f8 => 1
        }
    },
    f10 => 1
};

##</code><code>##

my $output = {
    f3 => 'important data',
    f5 => {
        f6 => 'more important data',
        f7 => {
            f8   => 'important data',
        }
    },
    f10 => [ 'important', 'data' ],
};

##</code><code>##

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;
}