/(?<foo_match>foo)(??{undef $+{foo_match}})/
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This is in line with other regex-related variables (such as the current match position), which can't be altered mid-pattern.

You could do something equivalent using embedded code and setting a variable rather than using named capture. However, that might not be the best idea.

In my mind, when a pattern starts having this much internal logic, the correct way to solve the problem is usually by breaking it down into multiple steps. As cool as Perl's advanced regex features are to play around with, just because you can do it all in a regex doesn't mean you should.