The maximum file size depends on your RAM size, XML::Simple creates a complex data structure, which in perl terms, will be much larger than the xml files size.
The other thing to note is that XML::Simple can be very slow on complex/large xml documents, so it may be that your browser is timing out, before the parsing is finished,

Id try running the script on the command-line to see if you get an "out of memory error" (from the OS), or to time how long the parsing takes.