```{index} single: lock_ref_keys_plus; Hash::Util function ``` ```{index} single: Hash::Util::lock_ref_keys_plus; Perl function ``` # lock_ref_keys_plus Lock the hash to the union of its current keys and a given extra list. ## Synopsis lock_ref_keys_plus($href, @additional_keys); lock_keys_plus(%hash, @additional_keys); # plain-hash form Equivalent to `lock_keys($hash, @additional_keys, keys %hash)` without having to list the existing keys yourself. Extra keys that are not yet present in the hash become hidden placeholders — they are legal to assign to later but do not yet appear in `keys`. Returns a reference to the hash.