Perl is quite efficient for an interpreted language, and it's much easier to write short, powerful code in. You can write your whole application in a day or two, make adjustments as necessary over the next few weeks, then when traffic starts to get high and efficiency becomes a concern, rewrite the least efficient parts of your application in a different language such as C++. Writing the whole thing from scratch in C++ would be hell.

Or you can just buy a block or two of memory, which is probably cheaper than rewriting the code in C++. Hardly any applications will need more efficiency than Perl / Mod_Perl can dish out, anyhow.