I think you're in for a good surprize there, you can even do

subset Even of Int where { $_ % 2 == 0 };

sub foo(Even $a) {say $a;}

foo(2); # prints 2

foo(3); # Constraint type check failed for parameter '$a'

XML parser defined using perl6 grammar rules: http://github.com/krunen/xml/blob/master/lib/XML/Grammar/Doc...

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

One of the other cool capabilities of perl6 is the rules system, expanding on regexs so you can work with full on grammars. The perl6 grammar, written in perl6:

http://svn.pugscode.org/pugs/src/perl6/STD.pm6

It's clear that doing it in an older syntax is clearly less elegant. However, it's very impressive though that the back ports of Perl 6 features to Perl 5 are proceeding relatively nicely though. The work on Perl 6 has definitely been a boon to 5.

That's my next area of focus for Rakudo optimizations.