if ($line =~ /^\s*:([a-zA-Z]+)\s*$/)   # new list of letters
  {     
        $master_letter_list = lc $1; ## Force all letter lists to LOWER CASE only
        %master_letter_freq=(); ########### ADDED ##########
        
        for (split //,$master_letter_list)
        {
            $master_letter_freq{$_}++;
        }
        
        print "master_letter_freq:\n";
        print Dumper \%master_letter_freq;  #####################
  }