See also the discussion of the  (?PARNO) construct in Extended Patterns in perlre. This is available from Perl version 5.10 onward and does not have the scary "experimental" warning associated with it. It is preferable IMHO to  (??{ code }) where it can be used.

I don't have time now for an extensive discussion of your original question, and I may not be the right person to deliver such discussion in any event. Perhaps someone else can jump in?