Perl XML findnodes Error

In my Perl Script I try to parse a XML File with XML::XPath. I do the following:

my $xml = XML::XPath->new(filename => "dat.xml");
foreach my $row ($xml->findnodes('/pack/data')) {
...
}

I get an error in findnodes, but I don't now, how to find it. The scripts just prints out "Getötet". I don't use perl die function.

The xml looks like:

<?xml version="1.0" encoding="utf-8" standalone="no"?>
<!DOCTYPE pack SYSTEM "qy.dtd">
<pack>

<data>
        <d0>88485488</d0>
        <d1>58915015</d1>
        <d2>56</d2>
        <d3>0</d3>
</data>

<data>
        <d0>88485511</d0>
        <d1>16023676</d1>
        <d2>56</d2>
        <d3>0</d3>

</data>
</pack>

How can I find the error? The XML is about 10 MB big.

Thank you !