// XS:
void my_callback(void *data)
{   
    pthread_mutex_lock(&event_queue_lock);
    
    push_onto_event_queue(data);
    
    pthread_mutex_unlock(&event_queue_lock);
    pthread_cond_broadcast(&event_queue_not_empty);
}

void get_next_event()
    PPCODE:
        pthread_mutex_lock(&event_queue_lock);

        while (event_queue_empty())
            pthread_cond_wait(&event_queue_not_empty, &event_queue_lock);

        void *data = pull_off_event_queue();
 
        pthread_mutex_unlock(&event_queue_lock);

        EXTEND(SP, 1);
        PUSHs(sv_2mortal(newSVpvn(data, ...)));

# Perl:
while (my $event = get_next_event())
{   
    event_handler($event);
}