Just a guess, maybe you should set walk/run speed of the mob at both 0 so that he can't move ? 
 
Also, maybe you should look at OP, or action (don't remember at all the exact name, i just know i did something with it) packets, get the command in PERL and execute it on mob spawn ? (sub EVENT_SPAWN) 
		
	
		
		
		
		
		
		
		
		
		
	
		
		
	
	
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