#!/usr/bin/perl
use strict;
use warnings;

my @genome = ('a', 'a');
my $i = 0;

my @seq = qw/ aa ag ac at ta tg tc tt ca cg cc ct ga gg gc gt /;
my ($tt, $ag, $ac, $at, $ta, $tg, $ga, $cg, $cc, $gc) = (0) x 10;
my %counter;
@counter{@seq} = \($tt, $ag, $ac, $at, $ta, $tg, $ga, $tt, 
				  $tg, $cg, $cc, $ag, $ga, $cc, $gc, $ac);				  

if (exists $counter{ $genome[$i] . $genome[$i+1] }) {				  				  
	++${ $counter{ $genome[$i] . $genome[$i+1] }} 
}