I once had a 1982 320IS with this problem.
The hoses to the fuel pump that's INSIDE the gas tank, or the breather hose from the tank to the filler pipe are probably seeping because of age. (Your car has TWO fuel pumps, the other one is near the rear axle along with the fuel filter which should be replaced every 30K miles!)
Take out the rear seat bottom. I believe there are two bolts about 6 inches above the floor. (If not, you may have to push the front of the seat cushion towards the rear to disengage a couple of hooks).
There's probably some soundproofing pads covering the passenger side fuel tank which is where the fuel pump is installed. If so, carefully remove the pad. With the seat bottom out, go fill up the tank, being sure that all your windows are open. (If you are a smoker, now would be a good time to quit!) You will probably find the leakage either as you fill the tank up, or right after you start the car. There's some good diagrams at Real OEM.com:
If you plan to fix this yourself, be very careful as parts of the fuel system from the 2nd pump to the engine are pressurized and will leak a lot of gas when opened. It is best to remove the fuel pump relay in the fuse box and run the car until it stalls out before doing any work. Also, be sure to not have a hot light bulb etc. anywhere that fuel could spill on it and explode. It is safer to diagnose the problem yourself and go to an independent repair shop, knowing what has to be done, than risking injury. The fuel hoses are cheap, but the car is a collector item.