I'm going to be running GM for multiple domains (and once I'm done customizing it will be running under mod_perl). What I want is to run a banner ad at the top of each page. The banner ad doesn't run under mod_perl yet, but I expect it will shortly, in case that needs to be handled differently. But, because I'm running multiple domains, what I've done in all my templates is include a few lines like this
<%include site_header.htm%>
<%include left_links.htm%>
etc.. in all my templates, so I can modify one file whenever I need to make a change, and not have to edit 30 or 40 templates for each domain. However, I want to call my banner from this new site_header.htm template that I created.

I'm getting lost in the code for where to do a system call to the banner ad program. I've been trying to go through Parser.pm, mostly in sub _parse_tags, but if anyone has any suggestions or code to share, I'd be most appreciative.