I don't know off the top of my head how to do this since I just started Perl and I haven't looked at this specifically but I am certain you can. What do you mean though? I don't really care that the Snort log is in snort.log. I more care that I grab everything that is being written to it (lets say check every 20 seconds with File::Tail) and parse it with Perl, and output it somewhere else.
He is suggesting that if snort were to be configured to send output its output to stdout*, your script could act like a filter (like grep), so you wouldn't have to use File::Tail and the process would be more reliable.
You'd still have to identify alerts, thought.
* — I'm not familiar with snort. This could be trivial or impossible.