I am not sure this will help but I do something that is similar. I have a C API which uses callbacks to process the results and I decided that the Perl binding would allow the user to specify Perl functions, so in my case I needed a C routine that would call a Perl function. My XS code looked like.

/* We have to bundle up enough of the XS environment to 
   be able to call perl functions from our C callbacks */

typedef struct
  {
    SV *sv;
    SV **sp;
  } SavedEnv;

static void
send_str(void *data,const char *str)
  {
    SavedEnv *env_ptr = (SavedEnv *)data;
    SV **sp = env_ptr->sp;

    /* NULL str means do a flush */
    if(str == NULL)
        str = "";

    ENTER;
    SAVETMPS;
    PUSHMARK(SP);
    XPUSHs(sv_2mortal(newSVpv(str,0)));
    PUTBACK;
    call_sv(env_ptr->sv,G_VOID | G_DISCARD | G_EVAL);
    FREETMPS;
    LEAVE;
  }

MODULE = Sgl        PACKAGE = Sgl        PREFIX = Sgl
PROTOTYPES: ENABLE

int
SglRawXmlList(fun,action_name,session_id,request_id,debug_level,pid_st
+r,wid_str,sid_str,cid_str)
        SV *fun
        char *action_name
        char *session_id
        char *request_id
        int debug_level
        char *pid_str
        char *wid_str
        char *sid_str
        char *cid_str
    CODE:
        SavedEnv env;

        env.sv = fun;
        env.sp = sp;

        if(*action_name == '\0')
            action_name = NULL;
        if(*session_id == '\0')
            session_id = NULL;
        if(*request_id == '\0')
            request_id = NULL;
        if(*pid_str == '\0')
            pid_str = NULL;
        if(*wid_str == '\0')
            wid_str = NULL;
        if(*sid_str == '\0')
            sid_str = NULL;
        if(*cid_str == '\0')
            cid_str = NULL;

        RETVAL = SglXmlList(send_str,&env,action_name,
                     session_id,request_id,debug_level,
                     pid_str,wid_str,sid_str,cid_str);
        OUTPUT:
        RETVAL
[download]

(Subset extracted for clarity)

My pm looked like

sub XmlList
  {
    my($callback,$action_name,%options) = @_;
    my($session_id,$request_id,$debug_level);
    my($projectID,$wellID,$setID,$curveID);

    if(!ref($callback))
      {
        my $ncallback = eval("\\&$callback");
        $callback = $ncallback if(ref($ncallback) eq "CODE");
      }
    if(ref($callback) ne "CODE")
      {
        carp "Must pass subroutine to XmlList\n";
        return;
      }
        .
        .
        .

    return RawXmlList($callback,$action_name,
                $session_id,$request_id,$debug_level,
                $projectID,$wellID,$setID,$curveID);
  }
[download]

As I said this seems to work for me and the places where it is different from yours seem insignificant

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