Pass variable to a scripted alert

Hi,

I would like to pass variable to run a perl script. I did it with fixed value and it runs well, but now i want to get Ip and user name from a search to run my scritp. I also did a alert with conditions to run the script and it works to. But i don't see where to send arguments ? Do i have to parse the field to extract the right value with regex into my script ? The documention here is not useful cause you can't see where and how to pass value. http://docs.splunk.com/Documentation/Splunk/5.0/Alert/Configuringscriptedalerts