Input:
<xxx>...</xxx>
<p>...</p>
<xxx>...</xxx><xxx>...</xxx>
<p>...</p>

##</code><code>##

Output:
<1><xxx>...</xxx>
<p>...</p>
<2><xxx>...</xxx><3><xxx>...</xxx>
<p>...</p>

##</code><code>##

   if(/<xxx>/){
    s/(<xxx>)/<$cnt>$1/;
    ++$cnt;
    }