Q1. How do I make the constructor return all three data structures, $hashref, @array, and $arrayref when invoked. Is it possible? I understand that only one of these three can be returned by the constructor and not all together. Correct?

A constructor is ment to return a reference to the just created instance of your class, which is of type "ABC".

Q2. I guess with $object1,$object2,$object3, I am creating three different objects. Right? Is there a way by which I create just one object of ABC and through this object I am able to manipulate data in all three data structures of, $hashref, @array, and $arrayref.

Yes, your object will hold those three data structures and provide access to them using methods (the oo term for funtions).

Q3. Do I need to create three different sets of get() and set() functions each to manipulate $hashref, @array, and $arrayref respectively by creating their objects first, ie $object1,$object2,$object3 ?

You could use the functions to manipulate your data structures through them instead of only retrieving them to set them again later. If you don't want / need this, you probably don't need OO for this task at all.

Q4. I understand I can manipulate $hashref, @array, and $arrayref only through access functions like get() and set() and not directly like $object1->$hashref or something like it, am I correct?

You should only access your data through methods. One idea/goal behind OO is to encapsulate your data. Simple accessors/setters/getters may just look like needless typing work, but imagine you want to change the way your data is stored at a later point. By using the encapsulation you can do so without having to change any code that is using your class (could be a lot!) by only altering your class.

Q5. (...) Perhaps, the nested structure might make the program slower, yes/no?

See Answer to Q2 and Q3 for the first part of your question. For program speed you should be aware that OO will most likely have a negative impact. The main purpose why to use OO is structure, maintainability, code reuse and the like over program speed.

Hope I could provide some useful hints.