There was nothing Barrera could do, Pacquiao was too fast for him to react to. In the first fight, Pacquiao beat the shit out of Barrera, in the second fight he stayed on the outside and used in and out movement and Barrera couldn't time him, it wasn't orthodox boxing, but in his own style Pacquiao was able to control the pace of the fight and land most of the meaningful punches that IMO is outboxing the opponent. Barrera to his credit fought well enough that Pacquiao wasn't engaging the same way he did in the first fight, but I think part of that was because Manny didn't have to.