I would also suggest not to introduce yet another alias but let the user decide which "CPAN-search engine" to use.
Along the line the Display Settings lets the user select the language preference for WikiMedia links, the user could select his/her preferred CPAN-search engine (cpan, metacpan, etc.). So the one and only cpan:// tag is dynamically mapped according to the users individual preferences.
That would leave 16k zombies (half ;-) alive.
Although this approach might cost some additional cycles. If one wants to add another level of complication, user preferences can be ignored if a node is older than July, 2011 or so.