I'd think the ASP route is more or less out: it's a pure Windows solution (from your post, it seems you want to run linux.) That being said, I'd say if you are a Windows person, and want to take what comes with it, ASP may be a good choice for the web part, you may need other Windows technology to do the index/searching part.

Speed/scalability should NOT be your concern for making this decision, period. It's pretty difficult to find sites for which perl, php or asp is not fast enough (maybe google, yahoo, ebay, amazon?). When speed/scalability does become a problem, it's inevitably the programming/programmer, not the programming language. That being said, if you choose perl, do try to use mod_perl, it's magnitude faster than perl/cgi.

Now PHP vs. perl. Since you're here, you must know all the good things about perl. PHP is a web centric language, it was especially invented for web programming, so it does make many web things easier, esepecially for beginners. After 5 versions, PHP5 is looking like a decent language, and I'm sure it will get better and better because of its large following. Therefore I don't think you should have much concern if you decide to use PHP for the web part.

However, there are very few decent sites where you don't need other backend processing, this is where PHP is not good: it's not designed for, few people are using it for that, which probably means it won't get better in that aspect (ASP has the same problem). If you want a language that can both do your web stuff and whatever else, then perl is a better choice.