I came across a similar issue recently. As tempting as it is to stuff all your data in the HTML, or in the query string, it's rarely the right approach - you add to your bandwidth and then have to parse it back out anyway.

What I did was put the data in a database (I was using a database anyway, so the connection overhead was already there) and in the HTML just passed a unique id I could use to extract the data on the other end.

If you're really attached to XML, you could dump your data structure to XML - there are several modules for this, XML::Dumper is one of many. Or check out Storable for a non-XML variation.

I can't say without knowing your data whether a file or database would be better for your purposes - it all depends on what kind of data, and how much. (Which is faster for you? TIAS). If you write to a file, don't forget about locking.