You can send data between scripts via the URL (your WDDX packets, for example), but as you've found, you eventually may run out of room (around 250 characters).

When you start needing to hold onto more info than that, you might want to store the info in temporary files, and just pass an encrypted session key in the URL.

The documentation for CGI.pm has some examples of maintaining state. Also the Mouse book (CGI Programming with Perl, 2nd Edition from O'Reilly) has a lot of great discussion on maintaining state.

There are as many strategies for maintaining state with CGI as there are "Ways to Do It(tm)" in Perl.