In a nutshell, LWP is a set of perl modules that provide an API to the web (you can find much more on it here - I thought it was a generic perl module, but I could be wrong in which case it shouldn't be too hard to get hold of, infact you can download a version of it from CPAN via that previous link).
You should just be able to place the script in the same directory as tac.cgi (/usr/local/nagios/sbin on my server), call it something like tac2.cgi, then do...
chown nagios:nagios tac2.cgi
chmod 775 tac2.cgi
...so it matches the permissions of tac.cgi. Once that's done, you should be able to call it up on your web-browser. If you're not sure of the url straight off, just open the original tactical overview page in a new window and change the url to tac2.cgi.
Oh, and make sure you change the following to accurately reflect the location of your perl executable and your nagios username and password
[Edit: Oh, and I'm a networking guy too, not a programmer. Sucks to be lumbered with monitoring stuff, eh...? !lol]