The last comment inside your "Readmore" section was:

it seems that it would be the same whether or not there was a '+' before the '<' or '>'

That's the problem! The man page for open only talks about using "+>" or "+<" as the "special 3-arg case with the third arg being undef" in order to establish read/write access to an anonymous temp file. This makes sense, because the temp file is going to vanish as soon as the file handle goes out of scope, and only the portion of the script where the handle is in scope can see that file, and write and then read data there. That's cool.

The man page does not describe any sort of "special 3-arg case with the third arg being undef" when there is just a bare "<" or ">" as the 2nd arg -- because this sort of usage makes no sense: why open an anonymous temporary file (non-existent outside the scope of the file handle, unknowable and inaccessible to any other process) for only read access, or only write access??? That's pointless!

So the fact that perl seems to be treating "+>" the same as ">" in this case (and likewise for "<") looks very much like A BUG.

Your discussion of the perl source code was a fair bit over my head (if you made any mistakes, I wouldn't know), but I thank you for it, just the same.