for ( my $loc = 0; $loc <= length($dna) - 10; $loc += $increment ) {

##</code><code>##

my $end = int((length($dna) - 10)/3);
for my $i (0..$end) {
   push @windows, substr($dna,$i*3,10);
}

##</code><code>##

my @windows = map { substr($dna,$_*3,10) } 0..int((length($dna)-10)/3);