[perl #131350] Useless 'Useless use' comes out of is tighter

[brian d foy]

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#`(
When I compile this code, I get the error:

    Useless use of constant value [ ] in sink context (lines 9, 9)

I don't think it was useless because it did what I needed.

I really wanted the ᔑ to be tighter then ., but that's not a real
operator and I ccouldn't use it to set precedence.
)

    sub prefix:<ᔑ> (Pair $p --> Pair) is tighter(&postcircumfix:<[ ]>) {
        $p.value
            =>
        $ = $p.key
        }