It did not work!

Given your problem description, I wouldn't expect it to.
From the documentation on cpan:

validate 
  eval { $xmlschema->validate( $doc ); };
This function allows to validate a (parsed) document against the given
+ XML Schema. The argument of this function should be a XML::LibXML::D
+ocument object. If this function succeeds, it will return 0, otherwis
+e it will die() and report the errors found. Because of this validate
+() should be always evaluated.
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Just in case, here's a link to the W3Schools page on namespace's
Perhaps all you need is a starter on what namespaces really do.
Then put something like this in your code (careful of the generic variables):

#Match Namespace
my $rdoc = XML::LibXML::XPathContext->new($doc->documentElement());
$rdoc->registerNs( ns => 'namespace' );
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To reiterate ikegami's comments: If you provide example's of what your working with - either sample code or xml - it will result in better responses to your question.