But the inner given would hide the topic of the outer given. There's only one $_ variable...

Also, Perl 6 does not intuit that foo() should be a method call just because you happen to be in the scope of a class that defines method foo(). (That's a thing I hate about C++, actually.) So if you want to call a method without specifying the object, you have to at least put in the dot:

     .foo()      # same as $_.foo()
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If you find yourself wanting to nest interface contexts, it's probably a sign that you should have designed the class with both interfaces in the first place. A class is the natural gathering point of interfaces (or roles, in terms of Perl 6's generalization of interfaces).