Rather than a hard-wired system you write yourself, you might want to get familiar with Net-SNMP. Using NetSNMP you can monitor a the local host OS, remote OSes, routers, switches, firewalls, and much more. You can then wrap around that in Perl with, for example, Net::SNMP.

As for a place to monitor the monitoring, there's always Mon, nagios, or Big Brother. There's also Argus, which is written in Perl itself and has a plugin module system.

I used to monitor DNS, SMTP, POP3, IMAP, RADIUS, HTTP, MySQL, and more servers for their server protocols, memory usage, disk usage, processor load, temperature, and network throughput with this combination. I also used Argus to monitor dozens of routers, switches, firewalls, and ISDN/dialup access concentrators that had their own SNMP implementations that came with them.

Argus can be configured to email you when very particular circumstances happen (like after a router interface shows down once on its 5-minute test, it'll test every 30 seconds until it's back up and email you if it's not up within 7 minutes of showing down). It handles a dependency tree so that it doesn't test devices it can't reach because of a failure in the middle.

Doing the work in Perl is a good thing, because Perl is well-suited to this kind of project. Since there's already a good solution written in Perl, perhaps you should take a look at it before reinventing this particular wheel.