Question

How to call function in one module to another module in verilog?

Answer 1

Sure, if you couldn't build up a hierarchy, HDLs would be rather limited.
For example, you could have:
module ic2323( input p0, input n0, input p1, input n1, input clk, output q0, output q1, output x ); // Instantiate a pn for p0, n0 and q0. pn pn_a( .p (p0), .n (n0), .clk (clk), .q (q0) ); // Instantiate another pn for p1, n1 and q1 pn pn_b ( .p (p1), .n (n1), .clk (clk), .q (q1) ); // XNOR the two outputs together assign x = ~(q0 ^ q1); endmodule