At this time, there is no built-in way to manage external lib directories outside of each Perl instance, but it is on my roadmap (after I get my RasPi work completed).
Personally, I like each instance to have its own installation of modules, but I digress. It's been requested of me to build in the ability that you're asking for, and as I said, it's on the roadmap. For now, you can do what you're doing, or shift out @INC (unshift @INC, '/path/to/libs'). Some modules will break, others won't.
If you want an easy way to install a module on all Perl instances with berrybrew and avoid using a single lib dir:
berrybrew exec cpanm Module::Name
That'll install the module on all installed Perl instances. If you have a module to install but it won't work on some version, I've added in the ability to exec on certain versions only (NOTE: no space between the perl versions!):
berrybrew exec --with 5.24.0_64,5.10.1_32 cpanm Module::Name
If you have any problems or issues, or want to discuss options on how you'd like to see a lib option be implemented, feel free to open an issue on the berrybrew link above (my github), or message me here and I can send over my email address.
update: To boot, if you're working with Strawberry Perl portable editions, you can incorporate them into berrybrew by copying/moving the instance directory into c:\berrybrew directory, and ensure that it is named exactly the same as one of the entries in berrybrew available. Thereafter, you can clone it to whatever name you want. I did not get as far in the Perl registration logic to allow just dumping any old directory into the berrybrew dir and have it auto-register, but that feature isn't far off either./update