No, backreferences are not allowed there. It could be implemented, but I don't think it comes up often enough to be needed.

However, if I can simplify your example even further and assume that the first (\d+?) is always of fixed length (say 4 characters), then the following will give you what you want: 

  unpack("a4/A*", $_)

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In reply to Re: Backref in a regex quantifier by sfink
in thread Backref in a regex quantifier by Anonymous Monk

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