Free of mistakes?
All the right notes and rhythms?
Correct fingering and pedaling?
Natural phrasing and breathing?
Everyone will have his/her own level of perfection in mind. As you think about the perfection, you also have to understand that perfect can rarely be achieved.
This realization often causes people to stall: "I will never achieve the perfect, therefore I shouldn't even try."
It's more productive, though, to figure out what would "ready" mean. If the piece is ready, you have to feel good when you play it in public.
Ready is not perfect but it has your permission to fly.