[Perl5-syntax] How to test for a role in a method signature

[Buddy Burden]

Guys,

Michael (Schwern) suggested I seek the collected wisdom of the group
here on this question.  Basically, what makes sense to me is something
like:

    {
        package Blargy;
        use Role::Basic;
    }

    {
        package Foo;
        use Method::Signatures;

        method new ($class:) { bless {}, $class }
        method doit ($bl does Blargy)            # proposed syntax
        {
            # whatever
        }
    }

    my $f = Foo->new;
    $f->doit(42);                                            # should
give run-time error because !(42 does Blargy)

(I used Role::Basic here just to take Moose out of the equation).  So
that's the syntax that makes sense to me, and I could've sworn it
worked that way in Perl6 too.  But now I can't find it in any of the
Synopses.

(Note that MXMS allows a "does" in the signature, but it means
something totally different (it's essentially a synonym for 'is' and
applies a trait to the parameter).)

So, the question is, does this syntax make sense to everyone?  It
seems intuitive to me, but intuitiveness is often in the eye of the
beholder, so it seems prudent to ask.  If this is _not_ a desired
syntax, what _would_ be a good syntax for run-time verification of a
method parameter (similar to how 'Blarg $bl' would verify that
$bl-isa(Blarg))?  And, just as a point of reference, does anyone know
how this will be achieved in P6?

TIA for your time.


            -- Buddy