Well, that's one way to go about it. For something like that, where the data element has some sort of restrictions on it, you're much better off using typed data for the element you need to have escape, and let the class the data is blessed into pitch a fit in the event an assignment to it violates the constraits on that data. Since typed data can overload assignment, it's an easy way to have action-at-a-distance validation of data going into variables.

That eliminates the need for proxy objects and suchlike hackery, and provides a cleaner interface. I expect someone (like, say... you? :) will come up with a constraints module such that you can say:

  my $foo  has constraint({length $^a < 12});
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To allow tagging on contstraint conditions. (Assuming, in this case, that constraint takes a list of closures which all must return true to allow the assignment) Modulo proper perl 6 syntax, of course.