If your HTML is actually XHTML-compliant, you could use XML::Twig to parse it, and then do something like this:

my @tagged_paragraphs = $twig->get_xpath('//p[string()=~/\[tag\]/');
my @texts = map { $_->text() } @tagged_paragraphs;
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Note that if you have p's in p's (e.g., "<p>some text<p>inner [tag] stuff</p>outter</p>", this may give you problems (you'll get both "some textinner [tag] stuffoutter" and "inner [tag] stuff", I believe).