Last I checked, IIS servers have a broken path info handling. CGI.pm attempts to fix this bug internally, but probably shouldn't be relied upon. As SSI does not allow you to pass in info to the script (at least, not to my knowledge) you might need to just switch to dynamically generating the page entire, using HTML::Template or the like.

Cheers,
Erik

Light a man a fire, he's warm for a day. Catch a man on fire, and he's warm for the rest of his life. - Terry Pratchet