Maybe it helps to consider code clarity algebraically. While it's possible to write 2x = x^2, if you want to solve the equation, you need to isolate your variables on one side or the other. Maybe I'm stretching to make a connection to idioms, but I find the second of these pairs much clearer:

$i = $i + 1;
$i++;

$i = $i + $i;
$i *= 2;
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Unfamiliarity with Perl's list-processing features does tend to obscure the intent of Ovid's code somewhat, but I can follow it without the comments. (I'd argue it would be clearer with a single comment that said "strip empty lines, wrap paragraphs marked by newlines in paragraph tags".)

I'm not completely satisfied with either -- it's not clear why "\r\n" is used instead of $/, though Ovid's has a distinct advantage of the <p>$_</p> construct.