... I just want to the number of the first column to minus the second column.

Note that the OPed statement
    $a=$tmp1[1]-$tmp1[0];
yields the result of the second column minus the first column. (Actually, it gives you the result of the second array element minus the first and we're assuming these elements are input data columns, but that's another question.)

Update: It just occurred to me that the OPed question might be properly paraphrased as "I want the number of the first column to be subtracted from the second column"; in that case, disregard the reply above.