I say he needs to prove it :) 
#!/usr/bin/perl --

use strict;
use warnings;

use HTML::Parser;

my $p = HTML::Parser->new(
    api_version => 3,
    default_h => [sub{print join ' | ', grep defined, @_,"\n" },"event
+,tag,text,"],
#    strict_names => 1,
    xml_mode => 1,
);


$p->parse(
'<boo><foo><shoo><Moo><COW></BOO>
<html><body>hi <br> <a href="1"> hello </a>
<boo><foo><shoo><Moo><COW></BOO>
</body></html>'
);

__END__
start_document |  |
start | boo | <boo> |
start | foo | <foo> |
start | shoo | <shoo> |
start | Moo | <Moo> |
start | COW | <COW> |
end | /BOO | </BOO> |
text |
 |
start | html | <html> |
start | body | <body> |
text | hi  |
start | br | <br> |
text |   |
start | a | <a href="1"> |
text |  hello  |
end | /a | </a> |
text |
 |
start | boo | <boo> |
start | foo | <foo> |
start | shoo | <shoo> |
start | Moo | <Moo> |
start | COW | <COW> |
end | /BOO | </BOO> |
text |
 |
end | /body | </body> |
end | /html | </html> |

[download]
Google suggests its xmllint that complaining.

In reply to Re^2: HTML::Parser and "Invalid foo tag" by Anonymous Monk
in thread HTML::Parser and "Invalid foo tag" by dragonchild

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
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