 Something that is perishable, which literally refers to a body that is capable of decay, is not something that's going to go to the same place that's imperishable, a place that's not capable of decay. You can't take a decaying body into a place where there is no decay. You can't take a terminal body into a place where there is no termination. Quite frankly, what Paul is saying here is that time for this body will run out. We cannot get into God's future eternity until it does.