Thinking about this. If you know someone is a perve this is what you do:
1) If they are local, report them to the local police
2) if they are not local, or international, then the police won't do anything, they will just refer you to the federal investigative powers. In the USA you can make an anonymous report online to the FBI.
3) You could always reach out to Anonymous hackers...they seem to be able to bring pervs to their knees quite effectively.
4) If you are too afraid to do anything just think about how many children you could save from horror and just find a bloody way to report it, even if you need to ask someone else to do it for you.