I am building a perl-based web client that retrieves web pages in the same manner a browser retrieves web pages, but my program is started from the linux command line.

You may be already aware of it, but the Lynx browser does exactly what you describe above. Moreover, there would be a variety of ways to insert the additional info you seek without redoing Lynx's functionality. (For instance, you could create a local static web page with frames. The top frame would display your info, the bottom whatever URL the user wanted)

If you are running Red Hat 5.1 Linux, you will find that right out of the box RH installs Lynx. (RH also installs an instance of Apache that starts automatically at bootup and is visible from Lynx as http://localhost.) Others can get Lynx here.