Perk is garbage.
Garbage that ranks 26th in the entire league in opponents' points allowed per possession.
26th isn't that great for a center that is meant to be a defensive specialist.
How about the net points for versus points against for his position?
Good points. Collins was even better (97th percentile), which I think is 15th or so in a league of 450 players. And Stiemsma was 29th in the league last year as of April. If most other players in the top 50 are defensive bigs, Perk might not be special. (Darko ranked out exceptionally on this metric too if you recall). There are some other issues too - I think last year Barnagni was top 20 and Tony Allen was something like 150th in the NBA overall.
In terms of team defense, OkC is about one point better on D with Perk on the floor - and they are also about one point worse on O. At least according to 82games.com (which I don't always trust).
The other interesting thing from 82games is that Perk's opponent PER is quite high at 17.1.
I'm not quite sure how to fit all of these metrics together, because they all have their limitations. I guess I'd view them as indicating that Perk is pretty good, at least, but I don't think they prove that he's a top defender at his position.