sub demonstration {
        return {
            demonstration_data => { demo_data => shift },
        };
    }

##</code><code>##

    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'
                                      }
            };

##</code><code>##

    sub foo {
        my $foo = shift;
        return
            $foo,
            [ $foo ],
            \$foo,
            sub :lvalue { $foo }
        ;
    }

    my ($noref, $aref, $sref, $cref) = foo('foo');

    my $dump = sub {
        print
            "\$noref: $noref\n",
            "\$aref : $aref->[0]\n",
            "\$sref : $$sref\n",
            "\$cref : " . $cref->() . "\n\n";
    };

    $dump->(); $noref     = 'bar'; # Changes the first value.
    $dump->(); $aref->[0] = 'baz'; # Changes the second value.
    $dump->(); $$sref     = 'zip'; # Changes the two last values.
    $dump->(); $cref->()  = 'zap'; # Changes the two last values.
    $dump->();

    __END__
    Long output, run it yourself.