If you hit a point where regex isn't working for you, consider one of the parsers on CPAN. For instance HTML::HTML5::Parser can make quick work of removing all the tags:

use HTML::HTML5::Parser;

my $parser = HTML::HTML5::Parser->new;
my $doc    = $parser->parse_string(<<'EOT');
<?xml version="1.0" encoding="UTF-8"?>
<title>Foo</title>
<p><b><i>Foo</b> bar</i>.
<p>Baz</br>Quux.
EOT
print $doc->textContent;
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Output:

Foo
Foo bar.
BazQuux.
[download]