Yesterday's Perl Cookbook "Recipe of the Day" seems quite relevant to your problem: http://www.perl.com/cookbook/perlckbk2/solution.csp?day=2

It comes from Perl Cookbook, section: 22.8. Processing Files Larger Than Available Memory.

There is a small example of how to use XML::Twig to read in only portions of a large XML file.

Note: the URL above will point to something different tomorrow.