Perhaps the real question you should be asking yourself is not how to do this but should this be done. Perhaps you have an innocuous need to open a program via a web form, but I know I'd be pissed as hell if a web page attempted this on my box.
Assuming the best intentions, the problem is most likely one of permissions. The web server (IIS??) has limited user rights. You may need to set up a virtual directory to the directory where outlook lives and set the most unrestrictive permissions possible. Not a very wise security practice, but probably possible.
update: I like derby's twist as it gets at the heart of what I was after. I was guessing that the goal might be some sort of a help page intended to get new users accquainted with their mail program, but that makes little sense since Perl would have to be installed locally for that to be feasible. If indeed the intent is to simply send mail via a web form, there are much, much easier ways to accomplish the goal as noted by derby.
I am what I read