Nice snippet, but a couple of problems: the outer lookahead needs to be //s, else eg:

  qr{(
    x
  )}x;
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Also, this will find parens in embedded code and comments and treat as captures. If that doesn't seem worth worrying about it'd be enough to add a caveat I guess, else I think you can mimic perl's simplistic parsing reasonable easily for the code (just count to the balancing close-brace). Comments may actually be the trickiest, since you'll need to know when //x is in force:

  qr{
    (?x: # (comment)
    )
    (?-x: # (capture) )
  }
[download]

Oops, another one: parens in [ ... ] should be ignored too; I'm not sure how easy those would be to parse, since not every ] closes the selection.

Hugo