How do you show the keys of a nested hash in Perl?

To show the keys of a nested hash in Perl, you'll typically need to traverse the hash structure recursively. Here's how you can do it:

1. Understanding Nested Hashes:

- A nested hash is a hash where some of the values are themselves hashes. For example, %hash = { 'level1' => { 'level2' => { 'level3' => 'value' } } };

2. Recursive Function:

- The most effective way to handle nested structures is by using a recursive function. This function will check if a value is a hash and, if so, call itself on that value.

3. Example Code:

sub print_nested_keys {
  my ($hash_ref, $prefix) = @_;
  $prefix //= "";
  foreach my $key (keys %{$hash_ref}) {
    my $value = $hash_ref->{$key};
    print $prefix . $key . "\n";
    if (ref($value) eq 'HASH') {
      print_nested_keys($value, $prefix . " ");
    }
  }
}

my %nested_hash = (
  'level1' => {
    'level2' => {
      'level3' => 'value',
      'level4' => 'another value'
    },
    'level5' => 'value5'
  },
  'level6' => 'value6'
);

print_nested_keys(\%nested_hash);

4. Explanation:

- The print_nested_keys function takes a hash reference and an optional prefix (for indentation). It iterates through the keys of the hash. If a value is a hash (checked using ref($value) eq 'HASH'), it calls itself with the nested hash and an increased prefix. Otherwise, it just prints the key with the current prefix.

5. Output:

- The output of the above code will be:

level1
  level2
    level3
    level4
  level5
level6

This approach allows you to display all keys in a nested hash structure, regardless of its depth. The recursive function ensures that all levels are traversed and their keys are printed.

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