my $line      = 'atccatccctttaat';
my @triplets  = unpack( '(a3)*', $line);
my @triplets2 = unpack( 'x(a3)*', $line);
my @triplets3 = unpack( 'xx(a3)*', $line);
# throw away trailing crud
pop @triplets  if $triplets[-1]  !~ /.../;
pop @triplets2 if $triplets2[-1] !~ /.../;
pop @triplets3 if $triplets3[-1] !~ /.../;