one approach would be to redirect to an html file with a query string after you've done all your processing in your perl such as:

$username = 'username';

print "Location: html.html?username=$username\n\n";
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from there, you can simply grab the variable on the html using this:

<script language="JavaScript"><!--
if (location.search.length > 0)
    eval('var ' + location.search.substring(1));
//--></script>
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