how can you say that kinetic energy in perfect elastic collision remains conserved ?

The statement of conservation for any case is defined for ideal cases.

So, ideally we define an elastic collision as for which the kinetic energy remains conserved.

 

Now, in practice, there is no ideal case present. Hence, even if the collision is perfectly elastic, the kinetic energy does not remain conserved.

 

So, our answer was for the ideal case definition.

The answer that you have presented is true in practical sense.