unable to parse xml file using registered namespace - perl

Question

I am using XML::LibXML to parse a XML file. There seems to some problem in using registered namespace while accessing the node elements. I am planning to covert this xml data into CSV file. I am trying to access each and every element here. To start with I tried out extracting attribute values of <country> and <state> tags. Below is the code I have come with . But I am getting error saying XPath error : Undefined namespace prefix.
use strict;
use warnings;
use Data::Dumper;
use XML::LibXML;
my $XML=<<EOF;
<DataSet xmlns="http://www.w3schools.com" xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.w3schools.com note.xsd">
<exec>
<survey_region ver="1.1" type="x789" date="20160312"/>
<survey_loc ver="1.1" type="x789" date="20160312"/>
<note>Population survey</note>
</exec>
<country name="ABC" type="MALE">
<state name="ABC_state1" result="PASS">
<info>
<type>literacy rate comparison</type>
</info>
<comment><![CDATA[
Some random text
contained here
]]></comment>
</state>
</country>
<country name="XYZ" type="MALE">
<state name="XYZ_state2" result="FAIL">
<info>
<type>literacy rate comparison</type>
</info>
<comment><![CDATA[
any random text data
]]></comment>
</state>
</country>
</DataSet>
EOF
my $parser = XML::LibXML->new();
my $doc = $parser->parse_string($XML);
my $xc = XML::LibXML::XPathContext->new($doc);
$xc->registerNs('x','http://www.w3schools.com');
foreach my $camelid ($xc->findnodes('//x:DataSet')) {
my $country_name = $camelid->findvalue('./x:country/#name');
my $country_type = $camelid->findvalue('./x:country/#type');
my $state_name = $camelid->findvalue('./x:state/#name');
my $state_result = $camelid->findvalue('./x:state/#result');
print "state_name ($state_name)\n";
print "state_result ($state_result)\n";
print "country_name ($country_name)\n";
print "country_type ($country_type)\n";
}
Update
if I remove the name space from XML and change my XPath slightly it seems to work. Can someone help me understand the difference.
foreach my $camelid ($xc->findnodes('//DataSet')) {
my $country_name = $camelid->findvalue('./country/#name');
my $country_type = $camelid->findvalue('./country/#type');
my $state_name = $camelid->findvalue('./country/state/#name');
my $state_result = $camelid->findvalue('./country/state/#result');
print "state_name ($state_name)\n";
print "state_result ($state_result)\n";
print "country_name ($country_name)\n";
print "country_type ($country_type)\n";
}

Answer 1

This would be my approach
#!/usr/bin/perl
use strict;
use warnings;
use XML::LibXML;
my $XML=<<EOF;
<DataSet xmlns="http://www.w3schools.com" xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.w3schools.com note.xsd">
<exec>
<survey_region ver="1.1" type="x789" date="20160312"/>
<survey_loc ver="1.1" type="x789" date="20160312"/>
<note>Population survey</note>
</exec>
<country name="ABC" type="MALE">
<state name="ABC_state1" result="PASS">
<info>
<type>literacy rate comparison</type>
</info>
<comment><![CDATA[
Some random text
contained here
]]></comment>
</state>
</country>
<country name="XYZ" type="MALE">
<state name="XYZ_state2" result="FAIL">
<info>
<type>literacy rate comparison</type>
</info>
<comment><![CDATA[
any random text data
]]></comment>
</state>
</country>
</DataSet>
EOF
my $parser = XML::LibXML->new();
my $tree = $parser->parse_string($XML);
my $root = $tree->getDocumentElement;
my #country = $root->getElementsByTagName('country');
foreach my $citem(#country){
my $country_name = $citem->getAttribute('name');
my $country_type = $citem->getAttribute('type');
print "Country Name -- $country_name\nCountry Type -- $country_type\n";
my #state = $citem->getElementsByTagName('state');
foreach my $sitem(#state){
my #info = $sitem->getElementsByTagName('info');
my $state_name = $sitem->getAttribute('name');
my $state_result = $sitem->getAttribute('result');
print "State Name -- $state_name\nState Result -- $state_result\n";
foreach my $i (#info){
my $text = $i->getElementsByTagName('type');
print "Info --- $text\n";
}
}
print "\n";
}
Of course you can manipulate the data anyway you'd like. If you are parsing from a file change parse_string to parse_file.
For the individual elements in the xml use the getElementsByTagName to get the elements within the tags. This should be enough to get you going

Answer 2

There seem to be two small mistakes here.
1. call findvalue for the XPathContext document with the context node as parameter.
2. name is a attribute in country no a node.
Therefor try :
my $country_name = $xc->findvalue('./x:country/#name', $camelid );
Update to the updated question if I remove the name space from XML and change my XPath slightly it seems to work. Can someone help me understand the difference.
To understand what happens here have a look to NOTE ON NAMESPACES AND XPATH
In your case $camelid->findvalue('./x:state/#name'); calls findvalue is called for an node.
But: The recommended way is to use the XML::LibXML::XPathContext module to define an explicit context for XPath evaluation, in which a document independent prefix-to-namespace mapping can be defined. Which I did above.
Conclusion:
Calling find on a node will only work: if the root element had no namespace
(or if you use the same prefix as in the xml doucment if ther is any)