My understanding is that mod_perlite would be a direct alternative to PHP. It would let you put perl code into your HTML files, which you could then upload to your shared-host web server (assuming they provided support via a mod_perlite apache module, similar to mod_php) and it would "just work".
Ideally, this should provide better performance that plain perl CGI, because the perl interpreter would only have to load once, when the web server starts up (just like PHP).
It would not be as powerful as mod_perl, but this is in some cases seen as a "good" thing because many (most? all?) shared-host web site admins see it as a security risk and thus don't provide it.
As a potential "PHP killer", mod_perlite has a lot of catching up to do. Most perl programmers have a low opinion of PHP and see it as an inferior programming language, and would be happy to make the switch if such an alternative were available. But I think PHP initially grew in popularity because it was considered easier to learn - a kind of "dumbed down" version of perl that non-programmers/hobbiests could use to quickly make "kewl" web pages, etc. So the notion that perl is "hard to learn" will probably remain a barrier to new user adoption.
IF the implementation hurts performance (equal or better to PHP is an obvious "must have") or IF it allows any security leaks (thus spooking would-be hosting providers) then it will never get off the ground. But I think it's a cool idea and would really love to see this succeed.