So, either you're using some other version (please state which), or the code you posted is not the code that generated that warning.
Or option 3: The point of the OP wasn't "why did I get this warning?", but, rather, "I made this mistake and didn't get a warning. Maybe it should issue one?"

In reply to Re^2: "Useless use of a constant" in grep block? by dsheroh
in thread "Useless use of a constant" in grep block? by perlancar

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