What MATLAB version are you using?
Newer versions actually support operations between e.g. a vector and a 2-d matrix if they have one dimension of the same length.
For your question, an easier work around is
k = 1;
prod(sum(mat1(k, 2:end) + mat2, 2));
you can make above code in loop to run through all rows. Should be ways to avoid this loop too.
