One of the things I’ve said I would do and never did is create a set of test suites for as many facts about a language as I know and then run it for every new release to see what I need to unlearn.
Most but not all of these were performance related. If it took a few days to run that’s fine. Major versions don’t come out that often.
<< is inplace append operator for strings/arrays, while + is used to make copy. So += will make new string & rebind variable