while(<DATA>)
{

{local $/='<a>';
   s!<a>(.*?),(.*?)</a>!<a>$1A$2</a>!gs;}

{local $/='<b>';
   s!<b>(.*?),(.*?)</b>!<b>$1B$2</b>!sg;}

   print;
}

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