OpenSCAD User Manual/The OpenSCAD Language

length = 10; width = 10; height = 15; fillet_radius = 2;

module roundedCube(size, radius) {

   minkowski() {
       cube([size[0] - 2*radius, size[1] - 2*radius, size[2]]);
       sphere(r=radius, $fn=50);
   }

}

roundedCube([length, width, height], fillet_radius);