Do you have to create it from scratch, or can you use an existing one?

directory.google.com is based on dmoz.org - the Open Directory Project. Unfortunately, the source to their engine is not available (but all the content is).

A quick search for dmoz on freshmeat.net showed several compatible projects, including one in perl.

I've never used any of them, so I can't vouch for them, but at least it's a pointer in the right direction.

You can also check the ODP Forums for ideas