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...
}