This example should do the trick. Note that DTD nodes don't appear in the XPath model, so I grep for those first.

use 5.010;
use XML::LibXML 1.70 ':all';

my $dom = XML::LibXML-> load_xml(IO => \*DATA);

my @nodes = (
    (grep { $_->nodeType ==  XML_DTD_NODE } $dom->childNodes),
    $dom->findnodes('//node() | //@*'),
    );    

foreach (@nodes) {
    say "Node name: ", $_->nodeName;
    say "Node path: ", $_->nodePath;
    say "---";
}

__DATA__
<!DOCTYPE foo>
<foo>
    <?processing-instruction?>
    <!-- a comment -->
    <bar attr="1" />
</foo>
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