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'