If we can't get someone better (and we probably can't, unless Dalembert is going to come around) then I would definitely take him.
We are paper soft in the middle on defense without Mozgov. If Mozgov gets in foul trouble, we have problems. We saw that right before the break.
Perkins isn't very tall but he has incredibly long arms. His standing reach is something like 9 foot 4 inches, which is gigantic. And he's massive, and he's a decent rebounder. And he's an enforcer, a goon. This team needs that, needs guys with an edge to offset some of our lack of size and grit.
I would definitely take him. The biggest problem with him is that, with him on the floor, you're playing 4 on 5 offense. But we don't really need his offense, we need his defense and his bulk. And he's won a title, so he's been through the playoff wars before.
Also, if we get him then we have no roster spots left. Which means that if we are going after a veteran point guard or looking at Ray Allen then someone or someones will have to go.
Perkins can still play, however marginally he plays. Haywood can no longer play. He is just a contract that is occupying a roster spot at this time.