That will work just fine and dandy as near as I can tell.
GREAT! - understandable! (in terms of what it does).
For something like this I would probably add a comment in the code for others as tip to why this "understandable thing" was needed. Not just for the future readers, but you may even forget "why" after a few years. That stuff has happened to me before! If the technique is so tricky that you had to think a lot about it, put in a comment.

But, yes,  "prefix_".$_ => 1 is "easy on the eyes" (and the brain)

In reply to Re^4: map problem by Marshall
in thread map problem by morgon

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