$array[$x]->{"foo"}->[0] = "January";
This is one of the cases we mentioned earlier in which references could spring into 
existence when in an lvalue context. Before this statement, $array[$x]may have been 
undefined. If so, it's automatically defined with a hash reference so that we can 
look up {"foo"}in it. Likewise $array[$x]->{"foo"}will automatically get defined 
with an array reference so that we can look up [0]in it. 
One more thing here.