i wrote a nagios plugin that validates the xml of a site.
the most important thing is the return codes.
Nagios expects one of 4 exit codes:
exit code status description
0 OK all good
1 warning working, but needs attention
2 critical not working correctly
3 unknown plugin could not determine the state
install the plugin in /usr/lib/nagios/plugins
make is executable
create the check..
Re^2: Help writing custom Perl nagios plugins