c:\@Work\Perl\monks\pearllearner315>perl -wMstrict -le
"my $seq = 'CCCAAAAAAAAAAAAAAAAAAAAAGGTTGGCCGGAAA';
 my @k_mers = $seq =~ m{ (?= (.{21} GG)) }xmsg;
 print qq{'$_'} for @k_mers;
"
'AAAAAAAAAAAAAAAAAAAAAGG'
'AAAAAAAAAAAAAAAAAGGTTGG'
'AAAAAAAAAAAAAGGTTGGCCGG'

##</code><code>##

c:\@Work\Perl\monks\pearllearner315>perl -wMstrict -le
"my $seq = 'GGGGGGGGGGGGGGGGGGGGGGGGGGGCCCAAAAAAAAAAAAAAAAAAAAAGGTTGGCCGGAAA';
 ;;
 my $rx_kmer = qr{ .{21} GG }xms;
 ;;
 my @k_mers;
 my %seen;
 ;;
 KMER:
 while ($seq =~ m{ (?= ($rx_kmer)) }xmsg) {
   push @k_mers, $1 unless $seen{$1}++;
   last KMER if @k_mers >= 1000;
   }
 ;;
 print qq{'$_'} for @k_mers;
"
'GGGGGGGGGGGGGGGGGGGGGGG'
'AAAAAAAAAAAAAAAAAAAAAGG'
'AAAAAAAAAAAAAAAAAGGTTGG'
'AAAAAAAAAAAAAGGTTGGCCGG'

##</code><code>##

c:\@Work\Perl\monks\pearllearner315>perl -wMstrict -le
"use List::MoreUtils qw(uniq);
 ;;
 my $seq = 'GGGGGGGGGGGGGGGGGGGGGGGGGGGCCCAAAAAAAAAAAAAAAAAAAAAGGTTGGCCGGAAA';
 ;;
 my $rx_kmer = qr{ .{21} GG }xms;
 ;;
 my @k_mers = uniq $seq =~ m{ (?= ($rx_kmer)) }xmsg;
 $#k_mers = 999 if $#k_mers >= 1000;
 ;;
 print qq{'$_'} for @k_mers;
"
'GGGGGGGGGGGGGGGGGGGGGGG'
'AAAAAAAAAAAAAAAAAAAAAGG'
'AAAAAAAAAAAAAAAAAGGTTGG'
'AAAAAAAAAAAAAGGTTGGCCGG'