The limit is function of the amount of memory you have on your system.

<plug type="shameless">If the whole document doesn't fit in memory, you can play with XML::Twig to load parts of it, process them and then free the memory before processing the next chunk. And of course, if you are used to XML::Simple interface, you can use the simplify method on any element to get the same structure that XMLin would have given you.</plug>