Here's the solution using plain WWW::Mechanize. It fails the XHTML test, because (I think) it uses HTML::TokeParser, and somehow misparses the Six link:

#!/usr/bin/env perl
use warnings;
use strict;

my $file = shift or die;
print "##### WWW::Mechanize on $file #####\n";
my $html = do { open my $fh, '<', $file or die "$file: $!"; local $/; 
+<$fh> };

use WWW::Mechanize;
my $mech = WWW::Mechanize->new();
$mech->update_html($html);

my @links = $mech->links();
for my $link (@links) {
    print $link->url, "\t", $link->text, "\n";
}
[download]

Since HTML::TokeParser and HTML::Parser even live in the same distribution, I'll look at a pull request to change the parser type to the one that works.

Update: The pull request