I've heard rumors that Perl 6 will solve this problem. Personally I don't think it's going to be all that helpful. New versions are released for a reason - leaving old code using old versions of their dependencies is only going to get you so far. Your example is a good one - if XYZ v1 and v2 are both being used to access the same physical database then you've likely got a problem that Perl alone supporting multiple version loading can't solve.

I haven't had a chance to try it yet, but I thought Erlang's facility for running multiple versions of code long enough to do a seemless cut-over sounded smart. Since everything in Erlang is a networked server the system starts up the new version and starts sending all new requests to it. Once the old version is done handling any lingering requests it gets shutdown.

-sam