use constant MOVEMENT => 3;
use constant WINDOWSIZE => 10;

my $dna = 'accatgagctgtacgtagcatctgagcgcgcatgactgtgactgacgtaggcagca';
my @windows=();
for (my $pos = 0; $pos <= (length($dna) - WINDOWSIZE); $pos += MOVEMENT)  {
     push(@windows,substr($dna,$pos,WINDOWSIZE));
}
                                                                      
print "@windows\n";