HK - that's excellent advice. The only aspect is the therapy part of 2e kids. Some kids do have to be in therapy regardless and they're not always going to like it.
My 2e ds7.5 has had years of therapies, interventions. I don't think there's been one therapist (pt, ot, speech, feeding, vision, neurofeedback, water) who hasn't noticed the perfectionism in ds and commented upon it. The behavioral optometrist even mentioned in a report that ds likes to "have complete control over his environment." Well, that's it with perfectionists. I come from a family who is loaded with them.
There are various gifted books which address the issue too - 101 Success Secrets for Gifted Kids, When Gifted Kids Don't have All the Answers, and even the older book, It's OK to be gifted and talented, which has some short stories for young kids. There's even a section in a book, You're Not the Boss of Me: Brat-proofing your 4- to 12-year-old, or various books on child defiance.
You could also try picking up the bestseller, The Feeling Good Handbook, and try some behavioral cognitive therapy. I'm sure there are perfectionist/anxiety charts online that you could try too. These are things that I'm going to try with my ds.
You can try messing up sessions and pointing out when you mess up.
With this topic, I often think of the difference between Paul McCartney and John Lennon. McCartney, from what I've read, is a serious perfectionist. He insists on perfectionism and thinks it's achievable. Lennon, by comparison, wanted to be perfect but would bag things if he couldn't achieve it. He'd be a little more acceptable of a piece if it didn't come out perfect while McCartney would wobble and be beside himself.
