[ModelSim] Passing real values to submodule

This looks like a ModelSim bug, although you probably do not want the code written this way anyways. Your real value would be converted to an integer and you would lose precision. You probably meant.

// submodule instantiation
submodule submodule_inst (
    .data    ($realtobits(a)), // IEEE 754 representation of a double precision floating point number.
    .out    (out_wire)
);
always @ * begin
    tmp <= $bitstoreal(data);  
end

module submodule(
    input real data,
    output real out
);