my $twig = XML::Twig->new(
    twig_handlers => {
        'attribdef' => sub {
            my ($twig, $elt) = @_;
            if ($elt->att('name') eq 'mtl_param') {
                # Do something.
            }
            elsif (...) {
            }
            # etc.

            $twig->flush;
        }
    }
)->parsefile('my xml file');
$twig->flush;