The script /usr/local/www/cgi-bin/monitorix.cgi contains the shebang line '#!/usr/bin/env perl'. However, Apache is started with a PATH of '/sbin:/bin:/usr/sbin:/usr/bin' and Perl is installed in /usr/local/bin, so env cannot find it. As a result, the CGI script cannot be executed.
Changing the shebang line to explicitly point at /usr/local/bin/perl fixes this.
There seems to be no way to change the PATH for Apache.