The main concern I would have is changes(*) that would cause a string result to have the dreaded "this is... UNICODE!" flag set. For hysterical raisins, Dreamwidth (as LiveJournal) wants all strings to be Unicode (using the UTF8 encoding) but with that flag clear, and clears that flag on all strings returned from code known to set it. When some new code returns a string with that flag set, all strings we combine it with (usually,the rest of the page this string goes into) get turned into gibberish, causing users and developers alike to go "double UTF?".

(*) in the interpreter itself, in core modules, or in any other modules Dreamwidth uses that need to be ungraded to run with the newer perl and core modules)