I havn't run the code, but it looks to me that your thread code

my $statusCallback = queryStatus();

is trying to use a sub located in main. I'm not surprised that it segfaults.

Is there any way you can write this so the XML code is all contained within the thread's code block?

UPDATE

Just for fun, I took out the while(1){} loop, and it does not segfault. I think that is where your problem may be. When that second thread in the while(1) loop gets created, it segfaults. Maybe you need to undefine your objects in the thread, reuse the thread (instead of making a new one), or figure out why XML::Twig isn't thread safe.

It will segfault on this too, as soon as the second thread is launched:

#while (1) 
#{ 
        my $thread = threads->new(\&querySystem);
        $thread->join;
        sleep 5;
#} 
        print "Starting second thread\n";
         $thread = threads->new(\&querySystem);
        $thread->join;
        sleep 5;
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