I am not sure it is fast but it works.
use Modern::Perl;
use Data::Dump qw/dump/;
my $seq = "CTTCGAATT"; # in this case length of 9
my @strings = $seq =~ /(.{3})/g ;
say dump(@strings);
Output:
("CTT", "CGA", "ATT")
CountZero
A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James