But... But... Then why did the double-encoding show up only in that one place??

Because the rest were ASCII characters.

use Encode qw( encode );
$str = '<p>foo</p>';
for (1..5) {
   print("$str\n");
   $str = encode('UTF-8', $str);
}
[download]

<p>foo</p>
<p>foo</p>
<p>foo</p>
<p>foo</p>
<p>foo</p>
[download]

I'm not sure about that. If I comment out the "binmode STDOUT..." in the OP code (having fixed all other encoding specs to "UTF-8" as described), I get "Wide character in print" warnings showing up in the error log

ARGH! CGI doesn't seem to be encoding. What's -charset for, then!? I need to look into this more.

By the way, <p/> makes no sense. <p/>text<p/>text means <p></p>text<p></p>text but you want <p>text</p><p>text</p> is what you want.