Let's first look at what kind of software the web and database servers are, what they need to run fast, and what implications they have on the rest of the system software.

The three important machine components are the hard disk, the amount of RAM, and the CPU type. Typically, the mod_perl server is mostly RAM-hungry, while the SQL database server mostly needs a very fast hard disk. Of course, if your mod_perl process reads a lot from the disk (a quite infrequent phenomenon) you will need a fast disk too. And if your database server has to do a lot of sorting of big tables and do lots of big table joins, it will need a lot of RAM too.

If we specified average virtual requirements for each machine, that's what we'd get.

An "ideal" mod_perl machine would have:

HD
Low-end (no real I/O, mostly logging)

RAM
The more, the better

CPU
Medium to high (according to needs)

An "ideal" database server machine would have:

HD
High-end

RAM
Large amounts (for big joins, sorting of many records), small amounts otherwise

CPU
Medium to high (according to needs)