First off, there's lots of perl editors floating around, but XCode isn't one of them. BBEdit is good if you have it, google around for others.
The plug-in format doesn't appear to have any documentation, but it's pretty easy to understand what they do. The script generates a plist containing information about the comic in question, which is used by iComic to display the comic. It's the responsibility of the script to understand how to extract the relevant information. Actually a pretty nice setup. I'd guess that you don't really have to use a perl script, but anything that's executable (though I'm just guessing...).
If you can read the html at vgcats and understand how to extract the relevant info, then you should be able to write your own plugin pretty easily.