It can already be perfect as in flawless, but then can be completed for another task. This is exactly what God. He became flesh, thus, became the necessary sacrifice for mankind.
Hebrews 5:9 - 9And having been made perfect (Gk. telios = complete), He became to all those who obey Him the source of eternal salvation, (NASB)
Sometimes, people unfortunately are unfortunately exposed to "straw men" concerning God.