ORIGINAL: Zeeb
I'm in agreement with zope_pope and Truckracer.
While the engine will probably run with it that close to the firewall and no hole, it certainly won't run well. There is actually a formula for figuring the necessary minimum clearance from the carb inlet to the firewall, but it basically boils down to you need at least the diameter of the carb intake i.e. a carb with a 5/8" intake would need at least a 5/8" clearance to the firewall and that's the minimum.
JMHO...
I guess I just prefer a nice large clearance hole so I don't have to worry about whether it is large enough or whether it interferes with airflow. I'm sure there are many ways of doing this sort of thing right. I have experienced carbs that are next to cowls where a hole was cut and it was not big enough. A larger hole solved the performance problem in these cases.