XML::Twig->new()->parsefile('my xml file');

##</code><code>##

my $twig = XML::Twig->new()->parsefile('my xml file');
# Use XPath-like expressions to find the nodes you want.
my @nodes = $twig->get_xpath('attribclass/attribdef[@name="mtl_Kdiff');
for my $node (@nodes) {
    # Process...
    # Example: $node->set_att('synthetic', 'false');
}

# Or navigate through references.
my @attribdefs = $twig->root->first_child('attribclass')->children('attribdef');
for my $node (@attribdefs) {
    # Process...
}

##</code><code>##

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;

##</code><code>##

my $hash = XML::Twig->new()->parsefile('my xml file')->simplify();
use Data::Dumper;
print Dumper($hash);