Why does your bearded dragon keep pooping on his perch and how can you stop it?

You can't 'stop a bearded dragon pooping' wherever it wants to !- They don't have a 'set area' to use as a toilet. As a reptile owner - it's YOUR responsibility to clean up after your reptile whenever they produce waste. If you don't like the idea of cleaning up after an animal - then maybe reptile-keeping isn't for you.