I'm sorry for misreading your answer (not enough caffeine).

I tried what I understood from you response and... it works! Thanks a lot for helping me. For any other person out there the xs that made it work was (note that the substantial changes are in the respond function, the rest was just because there was no more need to use the derived class):

#ifdef __cplusplus
extern "C" {
#endif

#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"

#include "ppport.h"

#ifdef __cplusplus
}
#endif

#include <string>
#include <libaiml/core.h>
using std::string;
using aiml::cCore;
using aiml::AIMLError;

MODULE = AI::AIML               PACKAGE = AI::AIML

cCore *
cCore::new()

void
cCore::DESTROY()

bool
cCore::initialize(const char* file)

void
cCore::deinitialize()

long
cCore::get_error()

const char*
cCore::get_error_str(AIMLError error_num);

const char*
cCore::respond(const char* input, const char* username)
CODE:
  string out;
  if (!THIS->respond(input, username, out))
    XSRETURN_UNDEF;
  else
    RETVAL = out.c_str();
  OUTPUT:
    RETVAL
[download]

I obviously also changed the typemap accordingly (cAiml -> cCore).

Don't know if you know what AIML is (you can google that), but if you do, you may understand my happiness when I read this:

PERL_DL_NONLAZY=1 /usr/bin/perl5.8.6 "-MExtUtils::Command::MM" "-e" "t
+est_harness(0, 'blib/lib', 'blib/arch')" t/*.t
t/AI-AIML....ok 3/4# Returned: Hello there.                           
+       
t/AI-AIML....ok                                                       
+       
All tests successful.
Files=1, Tests=4,  8 wallclock secs ( 7.48 cusr +  0.16 csys =  7.64 C
+PU)
[download]

(notice the "Returned", as an aswer to "hi") =] Thanks a lot.