.XS file: 
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
#include <TestInclude.h>
MODULE = TEST        PACKAGE = TEST
        
void
C_function(iReturn)
        int           &iReturn
    OUTPUT:
        iReturn


C function: 
#include "TestInclude.h"

void C_function( int *iReturn )
{
    *iReturn = 5;
}


.H prototype: 
void  C_function( int *iReturn );