Is there NO way to capture all the matches to a numerically quantified subexpression?

Indeed, there isn't. The number of capture groups is set by the number of capturing parens in the regular expression^†.

This should be trivial, yet it seems to be impossible.

I'm glad someone thinks the code dealing with regular expression in perl is trivial. AKAIK, you're unique. I'd think that patches would be more than welcome.

^†Actually, since 5.10, the number of capturing parens is an upper bound, due to the (?|) construct.