There isn't a single Greek word that directly translates to "perfect form" in the grammatical sense. The concept of the perfect aspect is expressed through verb conjugation, not a single descriptive word. You'd need to describe it using words like
τέλειος (téleios - perfect, complete) and
μορφή (morphḗ - form, shape) in combination, e.g.,
τέλεια μορφή (téleia morphḗ) - "perfect form," but this would refer to a perfect *shape* or *appearance* rather than a grammatical perfect tense.
To refer to the *grammatical* perfect aspect, you would need to use a periphrastic description involving grammatical terms in Greek.