PERL LibXML reading xml file with namespace not reading the file

[umesh_epub]

Hi, I have to read xml file using PERL LibXML. I will read many xml files. But this file have some namespace below is my sample xml file.

<?xml version="1.0" encoding="UTF-8"?>

    <alto xmlns="http://www.loc.gov/standards/alto/ns-v2#" xmlns:xsi="
+http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http:/
+/www.loc.gov/standards/alto alto-v2.0.xsd">
    <Description>
    <MeasurementUnit>inch1200</MeasurementUnit>
    </Description>
    </alto>

Here is my Perl code to read the above file

use XML::LibXML;
$xml = 'test.xml';
my $parser =XML::LibXML->new();
eval{
$doc = $parser->parse_file($xml) or die;
};
if($@){
return;
}
else{
$v = $doc->findvalue('/alto/Description/MeasurementUnit');
print "value\t".$v;
}

But when i changed the xmlns=" to xmlns:x=" It will work. But my xml not contain xmlns:x this only it contains xmlns="value".
Please help me to solve this problem.
Thanks, Umesh