while(<DATA>)
{
   s!<a>(.*?),(.*?)</a>!<a>$1A$2</a>!g;
   s!<b>(.*?),(.*?)</b>!<b>$1B$2</b>!g;
   print;
}

__DATA__
This is to test.
<a>a,
b</a>
<b>a,b</b>

##</code><code>##

This is to test.
<a>a,
b</a>
<b>aBb</b>