That's great because I am developing an XML Validation tool using XML::LibXML.

# Updated with invalid tags instead of nesting.
use XML::LibXML;
use LWP::Simple "get";

my $dtd_str = get "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional
+.dtd";
my $dtd = XML::LibXML::Dtd->parse_string($dtd_str);

my $doc = XML::LibXML->new->parse_string(join"",<DATA>);

$doc->validate($dtd);

__END__
<p><nope>Oh, noes!</nope></p>
[download]

Time!

And thanks to the anonymonk above. I didn't know you could use "grep" on search.cpan.org. Nice.