Genome scan for 6mer frequency

Split the genome into 6 bp windows and calculate the 6 mer frequencies.

#!/usr/bin/perl

use strict;
use warnings;

my $usage = "Usage $0 <infile.fa>\n";
my $infile = shift or die $usage;

my $seq = '';
open(IN,'<',$infile) || die "Could not open $infile: $!\n";
while(<IN>){
   chomp;
   next if (/^>/);
   $seq .= $_;
}
close(IN);

my %six_mer = ();
my $length = length($seq);
foreach my $frame (0 .. 5){
   my $frame_length = $length - $frame;
   #print "Frame length: $frame_length\n";
   my $mod = $frame_length % 6;
   #print "Mod: $mod\n";
   $frame_length -= $mod;
   #print "Frame length: $frame_length\n";
   for (my $i = $frame; $frame<$frame_length; $frame=$frame+6){
      my $mer = substr($seq,$frame,6);
      #print "$mer\n";
      if (exists $six_mer{$mer}){
         $six_mer{$mer}++;
      } else {
         $six_mer{$mer} = '1';
      }
   }
}

print "$infile\n";
foreach my $mer (sort {$six_mer{$b} <=> $six_mer{$a} } keys %six_mer){
   print "$mer: $six_mer{$mer}\n";
}
print "\n";

exit(0);

Scanning chr6 of hg19:

NNNNNN: 3719950
aaaaaa: 373380
tttttt: 372667
TTTTTT: 184768
AAAAAA: 182652
aaaaat: 143055
attttt: 142671
ATTTTT: 133646
TTTAAA: 133284
AAAAAT: 133130
TATTTT: 130672
AAAATA: 129572
TTTTAA: 129570
TTAAAA: 129177
aaaata: 123528
tatttt: 123134
atatat: 119872
...