in reply to Possible to use an expression as the hash key with exists?

It's not possible. I see 2 options:

  1. When creating the %$hashref_isbn, also create an %incomplete_isbn which references an array containing the complete keys (or at least the parts completing the isbn)
  2. iterate through al complete isbns you have, searching for matches
s$$([},&%#}/&/]+}%&{})*;#$&&s&&$^X.($'^"%]=\&(|?*{%
+.+=%;.#_}\&"^"-+%*).}%:##%}={~=~:.")&e&&s""`$''`"e

In Section Seekers of Perl Wisdom