That's what I'm thinking. I think she can make herself immortal if she wanted to, but she just hasn't figured it out yet, or she won't because she might not be able to make Nicholas immortal also. We know she can sometimes see the strands of time, and decipher what they mean, so could she be more than just human(besides being immortal)?
We know she defeated Niccolo in combat, and he is one of the more powerful immortals, so Perry at least probably rates up there with the elders. I want to know what the true color of Perenelle's aura is, how strong she is, and if she can really defeat an elder in combat?
Reading what Moviebuff said about Perry using the Codex to further her lore, could she have studied the spells and lore from the Earthlords' that are in the Codex?
Nicholas and Perry both said that Marethyu had to teach them how to read the Codex, but when Josh goes up to his room, and takes out the pages, he runs his finger over the pages, trailing some of his aura across the symbols on the page, translating it into English. So how much could Nick and Perry really learn from the Codex if they had to have someone teach them how to translate it, when Josh can just run his finger over the text and it automatically translates into English for him to read?
If Perenelle did find some Earthlord Lore in the Codex, she could be only 2nd to the Witch herself in the knowledge of Earthlord Lore. So that may be one of the reasons so many Elder's(Elder's and Dark Elder's alike) fear her so much, because they know she knows stuff that they can't counter with their own type of magic.