A perfect child-porn filter that only filters child porn would be wonderful
No, it would not. It would mean that "ordinary people", the average internet user, would not be aware of pictures of child abuse. The pedophiles have access to them anyway, and with public opinion being totally unaware, they can happily continue to abuse kids and publish the pics on their secret proxied servers. The best thing to do is to show every horrible picture to the public, to journalists, causing public outrage leading to funds being made available to put the abusers in jail.
I'm one of the founders of melpunt, the Dutch equivalent of the Cybertipline and Internet Watch Foundation (that were created after our example). I say, don't filter, don't censor, just put the pedophiles away for life. The kids who will never ever get their life back, deserve that.