Q: What do you mean when you call Mary perfect?
A: Human perfection is not an easy concept to define. When used in regard to Mary it is closely related to Mary's principal mission as mother of our Redeemer and second person of the Trinity, Jesus Christ, human and divine. Her "perfection" is required in order to be able to be the best of human mothers for the Son of God incarnate.
What does it consist of? In the absence of sinfulness or plenitude of grace! Both sinlessness and plenitude of grace warrant the highest possible degree of human empathy with God. The person who is totally in God or God in her will never break this ultimate bond with God (sin) because she is filled with the love, faith and hope that comes from God and leads to him. Scripture does not say that Mary is without sin but it says the equivalent, namely, that she is full of grace. See the event of the Annunciation, Luke 1,28: "favored one" or "with grace filled." Also, Luke 1,42: "most blessed one."
Mary's human perfection refers to the common denominator and source of all perfection: our bond with God. In this bond is rooted what we call a virtuous life. Does this mean that Mary was a beauty queen or a computer whiz? No. As human beings we are limited, that is, we are creatures. The only real human perfection is in God, since it is in the relationship with Him that we overcome our limitedness depending of course on how readily we receive His grace.