What happen if I do: delete($hash{abc}{def});

I would hazard a guess that the hash key value pair you have specified would be deleted. What happens when you try it? (I did not.)

.. is there a short way to delete also the first level key ('abc' in the example) when deleted all second level keys under it.

Sure, I would use delete($hash{abc}) This would delete the top level and everything underneath it.

And now, an editorial comment ..

Please, I would strongly urge you (and anyone else) to go into the Perl debugger and try these things out rather than posting here. Wait until you've tried a bunch of things and are well and truly stumped. We love interesting, challenging questions. Questions that show no effort leave us uninspired.