If its leaking from the rear, check the gasket for the fuel pump assembly (which is under a plate) on the drivers side in the boot, behind the rear seats. Also the inspection plate which is on the opposite side to the fuel pump, also in the boot behind the seats. Also check all hose connections under the plates, to ensure its not a leak coming from there.

If its not that, I'd be checking the rubber lines that connect all the hard lines together (there are hard lines that go across the top of the tank and then again under the left hand side of the car, all connected by rubber lines).

If its not that, check your hoses and clamps on the fuel filter. I'm not sure if an NA Impreza has the same setup as a WRX/STi, but if it does...its a cylindrical looking black thing under your bonnet near the firewall on the passenger side...even a small leak here can cause a strong smell to come through the vents into the cabin.