sub demonstration {
        my $demonstration_data = { demo_data => shift };
        return {
            demonstration_data => $demonstration_data,
        }, sub {
            $demonstration_data = $_[0];
        };
    }

    my ($data, $modifier) = demonstration('foo');

    print Dumper $data;
    $modifier->({ a => 1 });
    print Dumper $data;

    __END__
    $VAR1 = {
              'demonstration_data' => {
                                        'demo_data' => 'foo'
                                      }
            };
    $VAR1 = {
              'demonstration_data' => {
                                        'demo_data' => 'foo'
                                      }
            };