My first law of Perl6 learning: Though shalt not conclude anything from the implementations.

Perl 6 is quite a complex language to implement, so you should really consult the specs.

Sadly, pugs didn't have many commits latetly, due to audreyt being not too well (and a few other reasons), and doesn't parse everything that the specs allow, and doesn't reflect the newest changes to the synopsis.

Since things like my $a is foo = 3; are allowed (where foo is an arbitrary trait), I guess your example should work.