Yep very scrummy! And its also been available (for methods) in Perl5 for a while as well: http://news.ycombinator.com/item?id=1040614

Here is the perl6 example using MooseX::Declare (http://search.cpan.org/dist/MooseX-Declare/) in perl5:

    use 5.012;
    use MooseX::Declare;

    class Foo {
        use MooseX::Types -declare => [ qw(Even) ];
        use MooseX::Types::Moose qw(Int);

        BEGIN {
            subtype Even, as Int, where { $_ % 2 == 0 };
        }

        method bar ( Even $a ) { say $a }
    }

    my $foo = Foo->new;

    $foo->bar( 2 );  # => prints 2
    $foo->bar( 3 );  # => throws error