factorial.h============================
double factorial_recursive_c(int x); double factorial_iterative_c(int x);

factorial.c============================
double factorial_recursive_c(int x) {     if (x < 2)  return 1;
    return x * factorial_recursive_c(x - 1);
}
double factorial_iterative_c(int x) {
    int i;
    double result = 1;
    for (i = 2; i <= x; i++)
        result *= i;
    return result;
}

==============================
$h2xs -n Book::Factorial -A -O -x  -F  factorial.h
add
'OBJECT'            => 'Factorial.o factorial.o', 
to Makefile.PL
$perl Makefile.PL
$make
$make test
$make install