In other words, you want to dump the elements that match XPath

//descriptors[@type="CTC"]/descriptor/mainterm
[download]

Using XML::LibXML, the syntax is something like

for my $mainterm (
    $doc->findnodes( '//descriptors[@type="CTC"]/descriptor/mainterm' 
+)
) {
    print $mainterm->toString();
}
[download]

XML::Twig would also be awesome here.

Update: Changed
//descriptors[type="CTC"]/mainterm
to
//descriptors[@type="CTC"]/descriptor/mainterm
as per reply.