Perry made two huge mistakes.
He had back surgery. I don't care how tough you are, meds and pain will make you hazy when you speak, and the debate format - standing - did him no favors.
He didn't play to his strengths. He isn't a debater, though he can hold his own. He is a retail politician, best at off the cuff talk and meet and greets. He's no Newt. That guy can turn a simple question into a thesis instantly. Perry is more the sort of guy you'd like to sit down and have a beer with.