Deprecated modules warn when they are used
chris fedde asks How do we know when a CORE module is deprecated?. I don't know what was there originally, but it set off a lot of fingerpointing and posturing, and nobody answered the Perl question for the rest of the universe that finds it that post through Google!
As of Perl 5.12, deprecated features and modules warn the user as they're used.
There's the deprecate module which provides this warning. A module is deprecated for at least one cycle before it's actually removed. That doesn't help much if you skip over several perl versions.
Module::CoreList tracks deprecated modules too.
And, as one commenter, mentioned, the fallback is for people to read the perldelta pages to find the changes between the version they are using and the version they want to upgrade too.
The information is out there, and as programmers, we have everything we need to play with it however we like. :)