I want to say that separating business logic and database access into modules that implement CGI::Application, and presentation with HTML::Template is already (logically) a three-tiered architecture.

This is true, and worth emphasising. A tiered architecture can be implemented in any language.

Is it possible, or even interesting to divide these into physical tiers? Placing the presentation logic on one server and the CGI::Application stuff on another?

It's certainly possibly. However in many instances it's more trouble than it's worth - since you add a lot of overhead in communicating between the different physical layers.

There's a recent onjava.com article that discusses this (in the context of PHP but it applies equally to Perl).

What Perl doesn't have is a single commonly accepted way of handling separation of physical tiers. But, with TMTOWTDI, why would you expect to have only one way :-)

For example, it's trivial to create stand alone application servers with POE, RPC::XML or SOAP::Lite. The amount of work involved is tiny. You certainly don't have to drop down to the socket-coding level.