There's not really anything to decide. Putting an array inside quotes returns a scalar with the array contents, so if $foo == [ 'a', 'b' ], then "@$foo" == "ab". The regexp searches that string for the answer, so if the answer given was "b", what were essentially doing is "ab" ~= /b/i. This will match, and the correct grade counter is incremented.
Those who know that they are profound strive for clarity. Those who
would like to seem profound to the crowd strive for obscurity.