use strict;
use warnings;

my $string = "CTCCGGATCTAT";
my $counter = 0;

$counter++ while $string =~ /..[GC]/gi;

print "Found $counter occurences$/";

##</code><code>##

use strict;
use warnings;

my $string = "CTCCGGATCTAT";
my %flag = (C => 1, G => 1, A => 0, T => 0);

my $counter = 0;
$counter += $flag{$1} while $string =~ /..(.)/g;

print "Found $counter occurences$/";