That's neat -- and it could be done like this to make sure that both "undef" and "empty string" are handled without warnings:

$_ //= "?" or s/^\z/?/ for ( $this_variable, $that_variable, $another_
+variable );
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(UPDATE: But I'm inclined to think that when there's a problem with too many long variable names, it might mean that those variable names really ought to be keys in a hash.)

(updated again to fix the mis-typed operator in the snippet.)