A hash of hashes consists of an outer hash and many inner hashes. To get the keys of the outer hash, you would use:[tt]
@k=keys %outerhash;[/tt]
To get the keys of an inner hash, you would use:[tt]
@k=keys %{$outerhash{'innerhash'}};[/tt]
$outerhash{'innerhash'} represents the value of an element of $outerhash, but that value happens to be a hash, so you can then cast it as a hash by putting it inside %{...} and get its keys.