... I want a generic fatal error logger to write out a nice HTML page, complete with necessary debugging details for the user to forward to the webmaster.

use CGI::Carp qw(fatalsToBrowser); would be a good starting point for development. This is also probably OK for an Intranet site, but if you put this live on the Internet, you may be exposing yourself too much, see Does fatalsToBrowser give too much information to a cracker?.