We just take His Word for it.
Actually, we can't define perfection, because we have never seen it. We live in a fallen, imperfect world where everything around us has the curse of God on it because of Adam's disobedience. The whole planet is riddled with death and decay.
Just to add petrol to the fire:
A leading geneticist has stated that according to genetics, if mankind was millions of years old, we would have become extinct long ago, because of the running down of the genetic code. According to his research, the current genetic state of mankind is consistent with having just two original parents around 6000 years ago, and that the way that the genetic makeup of mankind is degrading, mankind will be extinct within another million years.
He bases his research on animals which have become extinct apart from man's affect on the environment, and has discovered that most extinct species died out because they got to the point where their offspring's genetic code was so degraded through continual mutation until offspring become so deformed that they cannot survive and continue breeding. He says that this will eventually happen to human beings.
Even if there were thousands of humans existing that provided the gene pool for mankind, the eventual day when mankind will no longer be able to produce healthy surviving offspring will not be significantly further away into the future than if there were just two original parents.
I found that quite interesting, and it shows that our world and everything in it is subject to decay and death, and therefore was not the original perfect world that God created.