Do you just have a single vector b, and you want the absolute value of each component to be < 6 (note, I will use <=, because that is how CVX treats <). If so,
norm(b,inf) <= 6
If instead, you have m different 100 by 1 vectors, and you want the norm of each vector (column) to be constrained, you can declare B as a matrix variable and use norms.
norms(B) <= 6