How is it perceived to withdraw an application to graduate school?
As an undergrad at a large, prestigious university, part of my work study job was sorting the department's PhD applications as they came in and went through various levels of review. There was absolutely no penalty to withdrawing. They received thousands of applications and the sheer number of those reviewed means that faculty are unlikely to remember your specific application (since you said this was a top school, I assume the same is true for yours). Also, most of the shifting around was done by me, an office assistant. If you've merely applied, there is really nothing to worry about. Now faculty might remember you if you had some kind of outstanding or unusual experience, or were interested in a VERY narrow program (one sub-program had only 3 applicants...). But again, no one is going to be very invested in you or your application until they have admitted you, so the time to withdraw, if that's what you're sure you want to do, is now. No one is going to take it personally given the number of applications involved.

If you have already been accepted, you can consider a deferral, as mentioned above, but even if you just withdraw there is unlikely to be a penalty. People have many reasons to decline--family, health, accepting elsewhere, unique opportunities. If you feel the need to explain and are very concerned about future chances, then perhaps consider a thoughtful letter to anyone you spoke with who accepted you (perhaps you applied to work on specific research in a field relevant to a faculty reviewer) saying that due to circumstances you cannot accept this year, but intend to reapply in the future and hope you will have the chance to work with them when you reapply. Honestly, being accepted in the past would be a great thing to put in your reapplication.