I am planning to update this page to contain full installation and configuration of awstats for apache as I have time to. If you want to add something, go ahead. -th0th
mod_perl package is in repo extra. So you can install it using pacman.
# pacman -S mod_perl
Awstats package is available in AUR. You can use yaourt to build it or you can obtain tarball from AUR and build it yourself.
$ yaourt -S awstats