foreach my $key (sort keys %hash) {
    print "KEY: $key \n";
    for my $nested_key ( sort keys %{ $hash{$key} } ) {
        print "nested key: $nested_key \n";
    }
    print "-------\n";
}
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I think that (re-)reading perldsc would help your perl progress.

update: Who put that there? Superfluous line removed!