Works for me (ActiveState Perl 10.1 on Windoze).

Did you miss the syntax error in your code (missing ; after until)? Try use warnings;

Here is my C:

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

#include "ppport.h"

#include "const-c.inc"

MODULE = MyModule        PACKAGE = MyModule        

INCLUDE: const-xs.inc

int
RegisterCB (SV *SubRef)

    CODE:
    
    ENTER;
    SAVETMPS;
    PUSHMARK(SP);
    
    XPUSHs(sv_2mortal(newSVpv("Some event",0)));
    XPUSHs(sv_2mortal(newSVpv("Some pdata",0)));
    
    PUTBACK;
    call_sv(SubRef, G_DISCARD);
    
    FREETMPS;
    LEAVE;
    
    RETVAL = 1;
    OUTPUT:
        RETVAL
[download]

Here is the Perl (its a long time since I didn't use strict;):

use MyModule;
use warnings;

#shared scalar
$cb_done = 0;

#This cb would be invoked by the cloned interpreter in the
#C library.  This works fine.
sub cb_one
{
  ($event, $pdata) = @_;
  #This line is printed.
  print "event : ", $event, "\n";
  $cb_done = 1;
}

$status = MyModule::RegisterCB(\&main::cb_one);

do
{
   sleep (5);
} until ($cb_done == 1);

#This line is printed as $cb_done becomes 1 
#in the main of the perl script
print "CB was invoked : $cb_done\n";
[download]

and here is the output:

C:\gash\MyModule>test.pl
Name "main::pdata" used only once: possible typo at C:\gash\MyModule\t
+est.pl line 11.
Name "main::status" used only once: possible typo at C:\gash\MyModule\
+test.pl line 17.
event : Some event
CB was invoked : 1
[download]

Update: original code was not passing parameters to the callback.
Update 2: I hadn't freed the temporaries and closed the scope

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