I have suspected all along that Perry is wearing some sort of back brace while on stage because of his surgery and the way he stands.
It may not be a solid one, maybe just one of those flexible types you see on the guys stocking stuff at big box stores, but the way he stands and walks to a smaller degree seems like someone in pain or a with a brace that is limiting his range of motion some.
He arches his back (chest out), like he's stretching it.