There are a couple of ways of doing this. One way could be to build a 4-input XOR module, and then instantiate multiple copies.

module myXOR4 ( input a, input b, input c, input d, output f);

assign f = a ^ b ^ c ^ d; // ^ is the XOR operator

endmodule

// …

myXOR4 xor1 (A[0],A[1],A[2],A[3],out[0]);

myXOR4 xor2 (A[4],A[5],A[6],A[7],out[1]);

// etc

Another way would be to use a for-loop. This won’t work with all the cases, since you don’t have an evenly-divisible number of wires.

reg [4:0] out;

integer i;

always@(*) begin

for (i=0; i<4; i=i+1)
out[i]=A[4*i] ^ A[4*i+1] ^ A[4*i+2] ^ A[4*i+3];
out[4] = A[18] ^ A[17] ^ A[16];
end
There are some other tricks (like 0-padding the upper-bits of A to make it evenly divisible, which makes the loops simpler) but they'll all end up with the same result, so this should be enough to get you started.