Romney is a well-known flip-flopper. If he thinks he needs to change his position on an issue to win a race, he will — and he will vigorously attack people for holding the positions he used to hold.
Perry is different. When he thinks he has to change a position, he will, too — but his temperament apparently does not allow him to follow through. Read his debate answer on the HPV-vaccine mandate, and you would never guess that he had already reversed himself on the issue. (“That particular issue is one that I readily stand up and say I made a mistake on,” he said in August. “I listened to the legislature, they said that was not going to occur, and I agreed with their decision. I don’t always get it right, but I darn sure listen.” Nothing like that the other night.) Clearly he thinks he was right the first time.