I can't think of any real difference. "Perhaps" sounds more formal than "maybe", so you will tend to hear people say "maybe" more often, but in most situations you can use either one and be understood. The only situation I can think of in which you can only use "maybe" is if you're invited to something and you're unsure whether you can go. Then you can sometimes says "I'm a maybe" or "put me down as a maybe", but those would be very informal phrases. (An example of this is when you're invited to an event on Facebook. You can't choose "going", "not going", or "maybe".)