The general solution for lvalue subroutines is to use Tie::Constrained.

my $checked;

tie $checked
    => 'Tie::Constrained', sub { defined $_[0] }, $INIT_VALUE;
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The real answer is that you can't return variables. You can return values. The variable being assigned to already exists outside of the function and is a receptacle for the value that is to be returned by the right side of the assignment operation. So tough luck. You could write some compiler macro, I suppose, if you wanted.