$line = 'atccatccctttaat';

push(@triplets, [ substr($line, $_) =~ /(.{$_})/g ]) for 0..2;

require Data::Dumper;
print(Data::Dumper::Dumper(\@triplets));

__END__
output
======
$VAR1 = [
          [
            'atc',
            'cat',
            'ccc',
            'ttt',
            'aat'
          ],
          [
            'tcc',
            'atc',
            'cct',
            'tta'
          ],
          [
            'cca',
            'tcc',
            'ctt',
            'taa'
          ]
        ];