It's been over 5 years since I worked with Nagios, but what I remember of it is that writing plugins is easier than trivial (of course, collecting the data you want to monitor may be difficult). All a plugin needs to do is writing 4 lines to STDOUT. It's trivial enough that for some plugins, Perl would be overkill: a few lines of bash will do.
in reply to Pure Perl Big Brother and Nagios clients: ideas? code bits?