If the Bible is the perfect word of God why do christians no longer practice the laws of the old testament?

I hear people say that the blood sacrifice of Jesus did away with the old law. If that is the case why keep the 10 commandments? If the 10 commandments are an exception what about all of the other laws? How can you tell whats fufilled and what is not? I can see the case for dietary laws being dispensed with, as there is that dream of Peter's mentioned where he is told it was all good to eat. But what about the parts commanding the killing of a bad child, or that women are unclean after child birth, and even more unclean if she has the nerve to have a baby girl. I am not trying to undercut your faith. I just hope to make you think about these things.