for my $key (sort keys %{$hash{A}{B}{C}{D}}, keys %{$hash{A}{B}{C}{E}) {
    print $key,"\n"; 
}

##</code><code>##

my @allkeys; 

push @allkeys, [$_, 'D'] for keys %{$hash{A}{B}{C}{D}};
push @allkeys, [$_, 'E'] for keys %{$hash{A}{B}{C}{E}};

for my $ref (sort {$a->[0] cmp $b->[0]} @allkeys) {
    my $key   = $ref->[0]; 
    my $which = $ref->[1];

    print "$key from $which hash.\n";  
}