1) I would identify through testing the absolute earliest version your code can tolerate. But then I would investigate (by testing against even earlier versions) what the impact would be of modifying code to push that limit backwards to allow as many sites as possible to be able to use the suite in question and make a feasibility decision as to what the earliest version should be.

2) A 'require ' + version (e.g. require 5.004;) prevents a script running in an environment earlier than the earliest version your policy then supports.

3) Module FindBin::libs offers automated ways to load lib directories into @INC/%INC without any minimised (hard-) coding (you have to tell it where to start recursively searching for modules).