You mean you're trying to load a very large XML stream from a web site? You seem to be assuming that the data is not too large to fit in memory...

Anyway, maybe you need to fork a separate process to use LWP::Simple, fetch data from the web site, and store it to disk in a file, so that the Tk script can keep moving along, responding to user input events like it normally does, and maybe check teh status of the child process and the status of the XML data file. Once it's ready, notify the user somehow, or just do whatever needs to be done with the data.