This term is used to describe a weapon or weapon upgrade that has the maximum bonuses available for its type. For example, a "hammer grip of fortitude" providing a +30 bonus to health would be considered perfect, while a +25 one would not. Also, a weapon with the maximum damage possible for that type is considered perfect. If someone refers to their perfect weapon, that implies that it has max damage as well as +15% damage while health is above 50% for a bonus.
Sometimes traders used the term Godly to describe perfect weapons.