The second way i tried but not correct (just i gave a try):

for (keys %po)
{
 
	$input = &process1 ($input) if (po{$_} == 1); 
	$input = &process2 ($input) if (po{$_} == 2); 
	$input = &process3 ($input) if (po{$_} == 3); 
	$input = &process4 ($input) if (po{$_} == 4); 


}