A talented debater does that by artfully taking a question he doesn't want to answer and skillfully twisting his answer to the talking point he really wanted to make.
The sad thing with Perry is that he's so bad at it, that sometimes, he doesn't even bother to try to twist his answer... he just simply goes straight into his answer as if the questioner asked something entirely different.
Its a huge turn off, too.
As an aside. I wish one of them, ANY of them would told the moderators that the US should use the same type of interrogation methods as their enemies use. If the enemy is torturing their captives, then why should the US do anything less. I would draw the line at hanging, burning and firing squads though (at least until they went to a military court). The US should not have their hands tied while the enemy gets a free pass. jmo