$h{x} is undefined. In order for something to be used in a %{ $something } construct, ref( $something ) eq 'HASH'. and ref( undef ) is undef, not HASH. While I'm not sure, I suspect the error occurs because %{$h{x}} is being passed by a prototype (namely (\%)) as a reference, as if you called keys(\%{$h{x}}), *I've forgotten the rest of my post, and I'm in a non-Perl mode now, so I can't complete it. Sorry*