How to add 'enable' to combinational logic

always @(out2)
valid_out=1;

This way, valid_out will stuck to one, because always @() is ignored in synthesis of combinational logic.

You should think about the exact purpose of valid_in, valid_out and enable with the present example. It's not obvious. You can sketch a table that shows the intended states of these signals. I guess, the said signals can't be but dummies in this case. So valid_out = valid_in is probably all you should do.