Fuel line cap problem/solution

That is part of the evap emissions controls and IS THE ONLY "functional" vent for the tank. The fuel caps have what is known as "pressure vacuum" reliefs. These are relief valves, NOT operational vents. Get into your shop manual and read the section on the evap in the emissions chapter as well as the chapter covering fuel tanks.

Generally, that line must be "up high" in the engine bay so that it will not siphon. Some guys plumb them into the non vacuum section of the PCV system, AKA whatever is leading to the air cleaner

My opinion, if you are going to pull or replace the tank, look up the very simple 69/ earlier vent system, which is a tube connection welded up high in the filler tube. They work well