use strict;
my $i = 2;

my @dna = split //, 'CTCCGGATCTAT';
my $counter = 0;
my $total = 0;

while ($i < @dna)     {
   if ($dna[$i] eq 'C')    {
          $counter++;
    }
   elsif ($dna[$i] eq 'G')   {
          $counter++;
   }
   $total++;
   $i += 3;
 }
print "Found $counter G/C of $total elements at a 3-position\n";