$ ./04-curl-multi-debug.pl 
TRACE 1
TRACE 2
TRACE 3
TRACE 4
GET_ONE 1 
GET_ONE 2 
GET_ONE 3 $$self: 1
GET_ONE 4 
GET_ONE 6 
GET_ONE 3 $$self: 1
GET_ONE 4 
callback function is not set
Operation was aborted by an application callbackTRACE 6: 'Operation was aborted by an application callback'