use strict;
use warnings;
my $string = "CTCCGGATCTAT";
my $sets = length($string)/3;                # we want to know how many sets of 3 there are
my $unpacker = 'A3' x $sets;                 # create our unpack template
my @dna = unpack($unpacker, $string);        # create the array of sets
my $counter = 0;                             # initialize the counter

for my $set (@dna) {                         # for each $set in @dna do...
  my $check = substr($set, 2, 1);            # get the 3rd char of the $set
  ++$counter if ($check =~ /[GC]/i);         # increment the counter, ignore case
}
print "$counter/$sets was either C or G.\n"; # print results