A four year old thread gets revived. The 1 1/2'" line is for gas to go into the tank. the 1/2" line lets air out as fuel pours in. So the gas expands as the air temperature goes from 60 to 90, they put a charcoal filter on the vent line so it allows the tank to breathe and the car doesn't smell like gas. Every ten or so times of starting the engine the solenoid closes and a vacuum is pulled on the tank to confirm that the fuel system has no leaks for 20 seconds.
Why would you want to drill hole in the canister? Find and fix the problem instead of breaking what could already be working. It's not that hard.

 

I've had this problem since summer. I finally got it solved and fixed today. There is an anti-siphon/roll over anti-spill device built into the plastic tank on 01' to 05' Blazers. It was stuck closed, with the filler hose disconnected from the tank. I could stick my finger in and feel the ball and cage but it wouldn't move down. The weight of the gas is suppose to push the ball. When I wiggled the cage around it would release from a burr or some thing and move down allowing gas to flow. When I released the cage it raised and stuck on the burr again. I used a large screwdriver to punch the device into the tank. At the gas station it took full volume from the pump.