# to turn your $str into an array
@array = map { vec($str, $_*6, 6) } 0..15;

# to turn an array of 6-bit items into a packed string
vec($str, $_*6, 6) = $array[$_] for 0..$#array;

# or simply to access the $i-th item in the $str, directly:
$read_item = vec($str, $i*6, 6) = $write_item;  # vec is an lvalue!