$x =~ s{(?<=<td>)         # Start match with open-tag
        (                 # Capture
          (?:(?!<td>).)*  #  Any number of characters that do not start an open-tag
        )3                # Close capture; match literal 3
        (?=               # Look ahead to match
          (?:(?!</td>).)* #  Any number of characters that do not start a close-tag
         </td>            #  then a close-tag
        )}                # End lookahead and pattern
       {$1a}x;