use strict;
use warnings;
use XML::Twig;

my $xfile = shift;
my $t = new XML::Twig(
   twig_handlers => {descriptors => \&desc}
);
$t->parsefile($xfile);

sub desc {
    my ($twig, $desc) = @_;
    if ($desc->att('type') eq 'CTC') {
        $desc->first_child('descriptor')->first_child('mainterm')->pri
+nt();
    }
}
[download]

Please post valid XML.