I get knocked for not being 'perlish' enough, but I am going to throw my two-cents in anyway:

It seems like switching namespaces would always be a bad idea (confusion, readablility, tracablility etc). While this might be a quick way out of a design-time roadblock, I think you are on the right track when you say "I'd prefer to have the package do what it does and not have to worry about add-ons that change its behavior"

You might consider if there are some minor refactoring to other parts of your project that can could suggest a better design.

With that said, sometimes we have to remember that it is not a perfect world and we have to make things work no matter what it takes.

Good luck.