XS is like another mini language. First you need a C compiler, actually you need the same compiler used to compile your Perl. So, on Linux is easy since is always with GCC, but on Win32 you will need Microsoft Visual C++ 6+ (not free) if you got Perl from Active State, or MingW, that is GCC for Win32.

Than you need to learn how Perl works with C and the resources for that (perlapi). So, take a look in the docs: perlapi, perlguts, perlxstut. All of that you can find better at http://www.perldoc.com.

If you already know C, I think that the best way to you make XS functions with Perl is getting as examaple the sub{C} methods that you can find in the sources of AI::NNEasy. Because this functions have a Perl and a C version, so you can compare them and understand better what the C macros does. Also the Class::HPLOO syntax is easy like sugar, since you only need to write the function, all the rest Class::HPLOO will write for you, converting to Inline::C, than Inline will convert to XS, and from XS is with Perl. So, the code below is 100% complete, you just need to have a C compiler well installed:

use Class::HPLOO ;

class Foo {

  sub[C] int add( int x , int y ) {
    int ret = x + y ;
    return ret ;
  }

}
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Now you just need to run the code above, and the rest is automatically.

Other thing, your module is inside AI::NNEasy starting from the class AI::NNEasy::NN. So, if you move this class (and the sub classes sinde AI::NNEasy::NN::*) to your project you will have your modules with the resources that I have added to it. I let it separated to be more easy to you to get back the work that I did, so be free to use it. ;-P