I don't think being a Perpetual is a real thing. I think its their term to describe being an immortal. Grammaticus describes himself as being "artificial" while Ollie is an organic perpetual. Kharn however is not a perpetual. He has the favor or Khorne and therefore is there is warp power that is sponsoring his vitality. The same is true of Lucius who is more of a parasite than an immortal. I don't believe that the Emperor is a human being in the sense as we perceive it. I think he is a power that inhabited a man or a man who evolved into a power. His physical appearance is based on perception and not reality which Lorgar described as "facelessness."
While being a perpetual has not been defined, I think it is a general term for immortals but this does not describe a lot of the immortals we see. While I believe the Saints are perpetuals and so is Ollie, that's it. The Emperor is something else and I don't believe the term human describes it. The fact that creatures from the warp that have observed and focused on humanity for countless eons describe him as not human is extremely telling. Creatures bred from human emotion would know a human when they saw one because of possible possession, summoning, and seduction potentials. They look at the Emperor and see something else entirely than a human and I don't think it's just the power. He possessed Malcador to speak with his son Corax. His face and body take different shapes and sizes to where his own genetic progeny can barely perceive his true visage. I'm waiting for the novel or codex that explains that.