Perfectionism does not imply that a person is perfect, it merely means that they have a bit of a compulsion to try to do things perfectly - which of course presents problems because they know very well that this is impossible.
Impostor syndrome comes about when one gets into exclusive company and feels that they don't really belong there - that they aren't good enough or smart enough et cetera. It is a state of disbelief that others would consider you to be good enough to be around them.
Of course, as the Dunning-Kruger research shows us, it is quite common for smart people to underestimate themselves, just as stupid people overestimate themselves. A smart person understands their own shortcomings and failures and feels as if they should not really be among smart and successful people. But they also tend to overestimate the abilities of others, finding it impossible to really imagine how dumb some people can be. Surrounded by smart people, therefore, they see their strengths and underestimate their weaknesses, and by contrast see themselves as lacking: voila! impostor syndrome.