This may not be the issue in this particular case, but it is an issue that you should address in your code: Always check the return values of system calls.

What does that mean? It means, when you ask Perl to open a file, you should check to be sure that the file actually got opened. Here is how that is often accomplished:

open OUTFILE, '>', 'pretty.xml' 
    or die "Couldn't open pretty.xml for output:\n$!\n";
# ... and later...
close OUTFILE 
    or die "Couldn't close output file, pretty.xml:\n$!\n";
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Also notice, within your open command you are using the concatenation operator, '.', when it's actually better in this case to use the comma.