All these problems are the fallout from the design mistake of considering regexen to be strings. In Perl 6 they are not strings, but merely programs in a specialized language that doesn't have to understand backreferences because it can get by with understanding variables, which include variables bound to earlier matches. Thus in Perl 6 you can just say

    / (\d+) x**{$0} /
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to do this very naturally. Also note that no recompilation of the closure returning the repeat count need be done here, since the closure is also real program and compiled only once. The only thing that changes is the binding of $0. (The numbered captures start at 0 rather than 1 in Perl 6 for consistency with the array view of the same data.)