$_='$0<paragraph>$1<paragraph>$2</paragraph>$3<paragraph>$4<paragraph>$5</paragraph>$6</paragraph>$7</paragraph>$8<paragraph>$9</paragraph>$10<paragraph>$11</paragraph>$12
';
($re=$_)=~s/((<paragraph>)|(<\/paragraph>)|[^<]+|.)/${[')','']}[!$3]\Q$1\E${['(','']}[!$2]/gs;
$re=join"|",map{quotemeta}(eval{/$re/});
s{($re)}{local $_=$1;s#</?paragraph>##g;$_}eg;
print;