Most carbs have a vent hole above the bowl's top level.Generally on flat ground with the `ped pretty well level, the gas should shut off BEFORE it reaches the top of the bowl.
If it leaks out the top of the bowl, the needle valve is probably letting fuel seep thru.(I know Bings are different so wait for a Puch mechanic to clarify here.)
Temporary fix? Shut your petcock EVERY time you stop for over a few moments.
Permanent fix? New needle valve and clean the needle valve's seat and check that float has not gotten fuel into it,making it NOT FLOAT.