if (exists $hash->{A}->{B2}->{C}) { ...

##</code><code>##

for my $k (keys %{ $hash->{A}->{B2} } ){ ...

##</code><code>##

   $hash->{'A'}->{'B2'}->{*}

#is actually Equivalent to
  my @subkeys = keys %{ $hash->{A}->{B2} };
#at least  - I think this is where you were heading...