#include <EXTERN.h>
#include <perl.h>
#include <XSUB.h>

XS(XS_Package_sub_name)
{
    ...[ Get arguments from Perl stack ]...

    rv = sub_name(arg);

    ...[ Return values on Perl stack ]...
}

int main() {
    static const char file[] = __FILE__;

    ...

    newXS("Package::sub_name, XS_Package_sub_name, file);

    ...
}