I'm not sure what you're after exactly, but I suppose you want the first "loop" to match when the third character is neither a 'c' nor a 'd'. Hence the condition should read

if ($third_char ne 'c' && $third_char ne 'd'){
  $part=substr($stri,0,2);
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Incidently, there are more elegant ways to check whether  $second_char is in @one. Have a look at grep, or alternatively, you can use a regexp.

  my $regexStr = join("|", @one);
  my $regex = qr/^$regexStr$/;
  ...
  if ($second_char =~ $regex) {
    ...
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Both approaches eliminate the need for the for loop.

Hope this helps, -gjb-