```{index} single: hidden_keys; Hash::Util function ``` ```{index} single: Hash::Util::hidden_keys; Perl function ``` # hidden_keys List the "hidden" placeholder keys of a restricted hash — legal keys that currently have no value assigned. ## Synopsis my @hidden = hidden_keys(%hash); A hidden key is one that is allowed by the hash's keyset but fails both `exists` and `defined` — typically the ghost left behind after a `delete` on a locked hash, or a key reserved in advance by `lock_keys_plus`. Assigning to a hidden key brings it into the visible keyset. Returns the empty list on an unrestricted hash. The feature is experimental and closely tied to the current restricted-hash implementation; do not build durable contracts on top of it.