(although I would have used the arrow de-referencing notation $hashref->{$_} instead of the $$hashref{$_} form)

Yes, right, that's actually what I would probably do also given the time. I typed my post in a rush before leaving for something else and used the dereferencing notation I still know best, but I have been trying in the last months to use more and more the arrow dereferencing notation, which is somewhat clearer. I am now using systematically the arrow notation for code refs and am trying to do it more and more for hash or array refs.