my $line = 'atccatccctttaat';
my %triplets;
for (0 .. 2) {
    @{$triplets{$_}} = unpack(('x' x $_).'(a3)*',$line);
    pop @{$triplets{$_}} if length($triplets{$_}->[-1]) != 3;
    print "Offset $_: @{$triplets{$_}}\n";
}