Says MeowChow:

It looks as if any complex sub-expressions /subroutine calls are evaluated in order, and variables/references are evaluated last.

That's very astute of you. It's not exactly what's going on, but it's very close. For simple variables, Perl pushes what is effectively a reference onto the stack. But for complex ecpressions, Perl must construct the new value and push a reference to that instead. So if $i = 2, then $i in the list pushes a reference to $i itself, but $i + 0 copies $i and pushes a reference to the copy of the 2. If you later change $i, the first 2 changes but the second one doesn't.

Your original example works the same way.

This could be considered a bug. It has come up on p5p before, but I don't remember what the outcome of the discussion was.

Contrary to what chromatic said, it is not a precedence issue. Precedence only affects parsing, not execution.

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