hidden_ref_keys

List the hidden placeholder keys of a restricted hash - legal keys that currently have no value.

Synopsis

my @hidden = hidden_ref_keys(\%hash);
my @hidden = hidden_keys(%hash);        # the \% form

A hidden key is allowed by the hash’s keyset but fails both exists and defined - typically the ghost left behind by a delete on a locked hash, or a key reserved in advance by lock_keys_plus. Assigning to it 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.