Genesis 17:1 And when Abram was ninety years old and nine, the LORD appeared to Abram, and said unto him, I am the Almighty God; walk before me, and be thou perfect.
The word for "perfect" in the original Hebrew does not carry the same meaning as it does in English. The word is tamim [ pronounced tawmeem ] and it means to be complete, whole, or sincere. To live a "perfect" life is to be sincerely dedicated to God and to learn dependence on His grace. It does not mean to be "faultless."
The same is true of the word for "perfect" in the New Testament which, in the original Greek, is teleios. Teleios carries the meaning of being grown up or spiritually mature.
We rejoice that, as we sincerely strive to obey God and the teachings of his word exemplified in the life of His son, our deficiencies, which are many, will be made good by the Father's loving grace towards us. It is this faith in God's goodness and mercy which allows us to live a "perfect" life in Christ. That absolute faith is the spiritual maturity which God counts as "perfection" even though we ourselves are far from perfect.
I hope you have found this helpful.