my $validator = XML::LibXML->new();
$validator->validation(1);

#test if the XML is well formed:
my $doc;
eval {
   $doc = $validator->parse_string($theXML);
};

if ($@) {
   # it's not well formed, your error handling code here...
}

#test if the XML conforms to the DTD:
eval {
   $doc->validate();
};

if ($@) {
   # it doesn't conform, your error code handling here...
}