It should look like this.
module multiplicador_3b(P5, P4, P3, P2, P1, P0,A2, A1, A0 , B2, B1, B0);
input A2, A1, A0 , B2, B1, B0;
output P5, P4, P3, P2, P1, P0;
wire s1,s2,s3,s4,s5,s6,s7,s8,s9,s10,s11,s12,s13,s14,s15 ,s16,s17,s18,s19,s20,s21,s22,s23,s24,s25,s26,s27,s 28,s29,s30,s31,s32,s33,s34,s35,s36,s37,s38,s39,s40 ,s41;
always @ (P5, P4, P3, P2, P1, P0, A2, A1, A0 , B2, B1, B0)
begin
s1 = (B0 & A0);
P0 = s1;
s2 = (B0 & A1);
s3 = (A0 & B1);
s4 = "0";
s5 = (s3 ^ s2);
s6 = (s5 ^ s4);
P1 = s6;
s7 = (s5 & s4);
s8 = (s3 & s2);
s9 = (s7 | s8);
s10 = (B1 & A1);
s11 = (B0 & A2);
s12 = (s10 ^ s11);
s13 = (s12 ^ s9);
s14 = (s12 & s9);
s15 = (s10 & s11);
s16 = (s14 | s15);
s17 = (A2 & B1);
s18 = (s17 ^ B1);
s19 = (s18 ^ s16);
s20 = (s17 & B1);
s21 = (s18 & s16);
s22 = (s20 | s21);
s23 = (A0 & B2);
s24 = "0";
s25 = (s23 ^ s13);
s26 = (s25 ^ s24);
P2 = s26;
s27 = (s23 & s13);
s28 = (s25 & s24);
s29 = (s27 | s28);
s30 = (B2 & A1);
s31 = (s30 ^ s19);
s32 = (s31 ^ s29);
P3 = s32;
s33 = (s31 & s29);
s34 = (s30 & s19);
s35 = (s33 & s34);
s36 = (A2 & B2);
s37 = (s36 ^ s22);
s38 = (s37 ^ s35);
P4 = s38;
s39 = (s37 & s35);
s40 = (s36 & s22);
s41 = (s39 | s40);
P5 = s41;
end
endmodule