Assuming the text in $_:

my $level = 0;
s{<h(\d)>}{my $r = "";
           if ($level < $1) {$r .= "<table>"}
           if ($level > $1) {$r .= "</table>"}
           $level = $1;
           "$r<tr><td>"}eg;
s{\n}{</td></tr>\n}g;
$_ .= "</table>" while $level--;
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