Both of my kids can at times display this type of perfectionism.
I tried to talk about it with them when they were NOT working on homework. When they actually were doing homework or drawing or anything that was likely to cause frustration, I just tried to be very careful about my comments, both positive and negative.
My kids at ages 8 and 12 both still want to do well on stuff. But they've both dialed back the anxiety and perfectionism they showed in their early school years.
It was as if at age 6, they expected that they would draw as well as the pictures they saw in books. They just didn't have a realistic idea of what they could do. As they matured, they realized that it was not realistic and became more satisfied with their own efforts.