Remember that it's only in the general case that you can't solve the halting problem. Many specific cases are easy to solve, and things can be designed to make it easier to solve those cases. In this particular case you'll note that, unlike Perl 5's dynamic scoping of the current successful match, Perl 6 makes $0 lexically scoped. That makes it pretty easy to tell how you've used $0 and its progeny within a particular block. Furthermore, the opaque $0 object is really not much more complicated than Perl 5's hidden match object. For most of the match, you've got to keep track of your state somewhere anyway. Perl 6 will undoubtedly do the same trick as Perl 5, and produce $1, $2, etc. lazily from the information.

Named captures are really no different in that respect. The name is really representing an offset into a capture table, and no hash is ever really built, except to hold the symbol at compile time. Perl 6 also will likely escape the $& problem by using copy-on-write semantics generally.