It doesn't look like there's a good reason for all three of those to be in the one template. Your example above would be fine and contain no HTML if you made those $html_output assignments call small templates to generate their HTML, rather than in-lining it. Or you could pull the header and footer stuff out into includes, so you'd just call different main templates for each case and they would all make include calls for the header and footer.
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Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:
<code> <a> <b> <big>
<blockquote> <br /> <dd>
<dl> <dt> <em> <font>
<h1> <h2> <h3> <h4>
<h5> <h6> <hr /> <i>
<li> <nbsp> <ol> <p>
<small> <strike> <strong>
<sub> <sup> <table>
<td> <th> <tr> <tt>
Snippets of code should be wrapped in
<code> tags not
<pre> tags. In fact, <pre>
tags should generally be avoided. If they must
be used, extreme care should be
taken to ensure that their contents do not
have long lines (<70 chars), in order to prevent
horizontal scrolling (and possible janitor
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