Re: Why am I getting wrong result in counting the number and kind of 2-letter in 3-letter words in a string?

[toolic]

Add this to your code:

use Data::Dumper; $Data::Dumper::Sortkeys=1; print Dumper(\%third);

Does this output look right to you?:

$VAR1 = {
          'AC' => 0,
          'AT' => 0,
          'C' => 4,
          'TC' => 0
        };

If not, I agree that Line 15 is not what you want. See also Basic debugging checklist. How about:

    $third{  substr($tri,0,1) . substr($tri,2,1) }++; # Line 15