my %counts;
++counts{uc($_)} for $seq =~ /.../sg;

for       my $l1 (qw( T C A G )) {
   for    my $l2 (qw( T C A G )) {
      for my $l3 (qw( T C A G )) {
         my $k = "$l1$l2$l3";
         my $v = $counts{$k} || 0;
         print("$k=$v;");
      }
      print("\n");
   }
}