MW2 Performance

Hey guys I purchased a 98 formula and noticed, when I shut the car off, or while its running for that matter, I can smell gas smell around the gas cap, I'm replacing the cap today, but I remember on my 95z that there was a little vent that is mounted in the left-rear wheelwell, from the gas tank (little white thing looks like a fuel filter with only 1 connecter on it) and I was wondering what that thing did again? Or if anyone else is having the problem and how they might have solved it.