Given this is Perl, I expect there is some way to do it...

But why? The whole point of these modules is to maintain that separation you are trying to break: It makes for easier to maintain, and less buggy, code. Trying to get around it should usually be the absolute last resort. (After even copy-pasting the module's code into a new module and writing an interface to do whatever it is you are trying.)

Please, explain what you are trying to do. I'll bet we can find a better way than modifying another package's private variables.