Perfectionism, which is not only a sin but also a disease, is basically the sin of pride. It assumes that we are not imperfect, but perfect.
Coniaris offers the most direct theological definition in the corpus, identifying perfectionism as a form of pride that denies the universal reality of human imperfection and imposes unrealistically absolute standards on self and others.
, Philokalia: The Bible of Orthodox Spirituality, 1998thesis