I suggest you take a look at `perldoc perlreftut`, the example they use is exactly the same as your (except the data ofc).

What goes wrong with @$Hash{ArrayInHash} is that Perl evalutes it as being: @{$Hash}{ArrayInHash} which means that it will try to dereference $Hash to an array, and then access the key 'ArrayInHash' in that array (which does not make sense as you can see).

What you should use: @{ $Hash{ArrayInHash} } (the whitespace is not required)