Ovid, Thanks for your response. To answer your questions, 1)I would want the substitution if I had "fo<some 200K length Tag>o", but NOT if I had fo<some tag>q<some tag>o. 2)I would also want "fools" to become "<bar>foo</bar>" The only reason the code you supplied would not work for me is that it would not do the substitution on "fo<some 200K length Tag>o" I would prefer it, if I were able to do:

while (my $token = $p->get_token ) {
  .....
  if ($token->is_text) {
        $text .= $token->return_text;
  }
}
$text =~ s/foo/<bar>foo</bar>/g;
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That's where I get stuck because I see no way to "merge" my "new" document that contains only <bar> tags AND the original text with the original document, which contained the original text and all the other tags. Hope that clarifies.