Quick question

I have a sub that takes in a ref to an array of arrays, and localizes. I later do something to a cell in the array of arrays to which that ref refers, and I get a "useless use of private variable in void context" error that I don't understand in this context.

sub mysub {
    $arrayref = shift;

    ...

    $arrayref->[$x][$y] = 1;

}
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Later:

Useless use of private variable in void context at mymodule.pm line 16
+6.
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