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I like the patch in principle, but I have a number of
additional concerns:
- there are no test cases
- the feature is declared experimental in perlre(1). Why?
- Shouldn't there be a semantic restriction that the back
  reference is only allowed if it points to a group that is
known
  to precede? I.e. is

  (X)|(?(1)Y)

  valid? If not, the restriction should be atleast documented, 
  but if possible, it should also be implemented.