my $dna = 'accatgagctgtacgtagcatctgagcgcgcatgactgtgactgacgtaggcagca';

my $increment = 3;
my @windows;
for ( my $loc=0; $loc <= (length($dna)-10); $loc+=$increment ){
    push @windows, substr($dna, $loc, 10);
}

print "$_\n" for @windows;