Well, I think I goofed on this one. From reading A12, it appears that anonymous classes are built and bound to objects that use mixins. I missed that point and now it seems like we're going to have inheritance problems again. When I posted a question about this to the Perl 6 language list, Larry Wall responded:
That's just a problem with run-time mixins in general if you can't know in advance that you're going to want both roles. That's why we encourage as much of that to be done at compile time as possible.
This does seem like it will cause more design issues, but Larry and TheDamian both had ways around this. They suggested strategies such as creating anonymous classes or anonymous roles that could disambiguate methods as needed. I guess that makes sense as magician thieves would want the magic users spell-casting ability but the thief's fighting ability and those seem like rules that might have to be decided at compile time.
You can read the Perl 6 language discussion here.