FreeBSD 4.9 (latest stable) scales excellently, and the 5.x series promises to tune many aspects, but isn't considered stable yet.

The Linux kernel had serious problems on SMP systems up to the version 2.4 series. 2.4 does well all-around, though suffers under very high loads. 2.6 catches up with FreeBSD in all regards and can basically be considered to be "playing with the big boys", a bit better here a bit worse there, but it's definitely a serious contender in all areas. Note that 2.6 is not available as a package in Debian stable; however, installing a 2.6 kernel is not an issue at all, anway, you just follow standard procedures..

A base install for both systems is pretty easy to accomplish (driver issues aside, of course..), and additional software isn't hard to install either, but actual administration does require reading. However, most of your effort at first will be learning where and how to find documentation; the learning curve is steep, but rapidly flattens once you get the big picture of the system and grok the inherent consistency of most things Unix.

And since you've done some Perl programming, you'll have at least vaguely heard of a number of Unix concepts, too, so that might ease the initial hurdle.