+=item PERL_PERTURB_KEYS
+X<PERL_PERTURB_KEYS>
+
+(Since Perl 5.18.0) Set to C<"0"> or C<"NO"> then traversing keys
+will be repeatable from run to run for the same PERL_HASH_SEED.
+Insertion into a hash will not change the order, except to provide
+for more space in the hash. When combined with setting PERL_HASH_SEED
+this mode is as close to pre 5.18 behavior as you can get.
+
+When set to C<"1"> or C<"RANDOM"> then traversing keys will be randomized.
+Every time a hash is inserted into the key order will change in a random
+fashion. The order may not be repeatable in a following program run
+even if the PERL_HASH_SEED has been specified. This is the default
+mode for perl.
+
+When set to C<"2"> or C<"DETERMINISTIC"> then inserting keys into a hash
+will cause the key order to change, but in a way that is repeatable
+from program run to program run.
+
+B<NOTE:> Use of this option is considered insecure, and is intended only
+for debugging non-deterministic behavior in Perl's hash function. Do
+not use it in production.
+
+See L<perlsec/"Algorithmic Complexity Attacks"> and L</PERL_HASH_SEED>
+and L</PERL_HASH_SEED_DEBUG> for more information. You can get and set the
+key traversal mask for a specific hash by using the C<hash_traversal_mask()>
+function from L<Hash::Util>.
+