I was aware of the INC object. Well, just because to be sure every now and again I check perldoc -f require, although in fact I usually stick with the "sub form of code-in-@INC". Now, indeed this kind of hackery is there for doing stuff like "extracting from tarball" (or retrieving over a network connection, maybe). In this case, if I "only" want to change a filename, or a relative path (e.g. Foo/*.pm to Foo-devel/*.pm) as shown in my previous example, then I may search regular @INC directories for the real file I want and return an open file handle to it. But that would amount to reinventing the wheel. And it wouldn't chain with other code in @INC. I may just want perl to do that for me instead...
Indeed agree that some care should be payed to a fallback mechanism and to informative error messages. But modulo these caveats, after all this is a somewhat advanced hackery and fundamentally adding the feature would amount to... removing a feature, precisely the one that makes the passed path read-only, so...
