I dont see any reason to "upgrade" a perl install. Do a parallel install of as many different versions as you choose. So long as they are in totally different directory trees they will never know the others exist. Then write and deploy your code for the new versions and leave the old code alone except if you have a really good reason to decide that it should be ported to a later perl.
I think the biggest issue in this line of discussion is that everybody assumes that you must "upgrade" Perl. Its not an OS, you can quite happily have loads of different versions available at the same time.