To clarify for Floyd,
Yes you can set up apache to run on only one IP#.
And then you could have multiple apache instances running on port 80. The limit is per port per IP#.
To clarify for welch,
We offer our clients the ability to run perl applications, and we have no problem running them the way DirectAdmin is currently set up. We do NOT currently support any of the others you mention.
What you can easily do is can make rather complex changes to the user-specific html include files, and chattr them so DirectAdmin can't overwrite them. If you want more, then you may be able to figure out how to do what you want within DirectAdmin but it's probably not a good choice, since DirectAdmin just wasn't designed for what you're writing about. In that case, then VPS would probably be a good way for you to go.
Jeff