My perfectionist is now 12. When she was a preschooler I signed her up for piano - something I knew it would take years to be really good at. Here was something that you could only do successfully with lots of practice, and furthermore, it was okay to do it "good enough" (of course it was important to find a piano teacher who was not going to demand perfection. We don't do recitals or competitions. We just play and learn music theory.) This also taught her empathy - this was what it felt like for other kids when they were learning how to read, or add, or whatever.
I also signed her up for soccer. I figured a team sport was an opportunity to run around, but alone you could not succeed. And you being perfect does not drive the perfect outcome.
We also celebrate Bs (okay, not very often) but when they come home we have a party - finally an area where she has an opportunity to learn something.
Discipline is another area to be careful with - she is far harder on herself than I would ever be. When she has crossed a line, I gently call her on it and then back away. After a short time I invariably have to encourage her to forgive herself and move on - she will otherwise stew on the point forever!
The most interesting thing to me was to learn what the results of perfectionism are in an older child (or even an adult.) Procrastination, not applying yourself and giving up. These are traits not easily overcome as an adult - they are worth getting after in our youngsters.
A child cannot learn to overcome perfectionism in an environment in which he is can be effortlessly perfect. He needs a challenging environment to begin with. Then he needs plenty of encouragement to take a risk and try something he may not quite understand. And then lots of applause for the babysteps he takes while he figures it out.
Good luck - it is hard work, but it is so important.
