I would say either that there's no such thing as the perfect girl, or that perfection is subjective or boring, or that the perfect girl for each person would be different, but that you haven't lived long enough/experienced enough to be able to know or describe your perfect girl, or that you simply don't have a concept in your head of a perfect girl. Answer with a non-answer.
Alternatively, you could ask her to clarify her meaning. Is she asking you to objectively describe a perfect person, or to describe the qualities/characteristics/traits you look for in a girl.
There are many ways to answer and no right or wrong answer. It's highly likely that she is just trying to figure out if she meets your criteria - if you don't want her to know your criteria or if you are worried about scaring her off then I would go with a non-answer. If you feel your criteria isn't too strict or obscure, a.k.a. most decent girls would fit the bill, then sharing probably wouldn't hurt.
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Into the dark...