"The (probable) reason that you see a different result from C, is that you are (probably) using single precision (floats) in your C code, whilst Perl uses double precision. (Probably:)"

This seems to be a good guess:

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char * * argv) {
    {
        double a = 40.88050;
        printf("%.3f\n",a);
        printf("%.17f\n",a);
        a = 41.78050;
        printf("%.3f\n",a);
        printf("%.17f\n",a);
    }
    {
        float a = 40.88050;
        printf("%.3f\n",a);
        printf("%.17f\n",a);
        a = 41.78050;
        printf("%.3f\n",a);
        printf("%.17f\n",a);
    }
    return 0;    
}
[download]

Prints:

40.880
40.88049999999999784
41.781
41.78050000000000352
40.881
40.88050079345703125
41.780
41.78049850463867188
[download]