Is there any unified list of officially undefined things?
That's the only one that comes to mind. It's documented (too specifically) in perlsyn under statement modifiers (my ... if ...;).
I know, for example, by luck, that $i++ + ++$i can officially do whatever it wants
That one's simply undocumented.
I wonder if it might be appropriate for the precedence table to mention this unexpected (by me) interaction?
I don't see why not, although I don't think it'll ever be an issue in legit code (but my brain is too slow to make sure of that right now). Maybe as a warning in the doc for the cond op.
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