($REF_array)=@_;
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That makes a copy of the array. You are no longer assigning to the references to the original arguments, and thus your values are lost once you get out of the subroutine...

Technically that makes a copy of the array reference. The @Array = @{$REF_array} is what makes a copy of the array referenced. The rest of your diagnostic is correct (the original arrayref is untouched by modifications to @Array). </pedant>