The default is not eating meat because the default is not eating anything. Humans must take a positive action to eat something.
Generally perfect duties in the Kantian sense take the form of "thou shalt-nots", so a positive action you shouldn't take, like killing a random stranger or some such. This is contrasted with imperfect duties, which are things that are merely virtuous to do - like using your time & resources to help the less-fortunate, for example.