Looking around a bit, there doesn't seem to be any reason not to use nfreeze instead of freeze (see e.g. http://blogs.perl.org/users/ovid/2010/10/storable-freeze-versus-nfreeze.html).
As Mike says, it is only used in two modules:
wwadmin@wwserver:/opt/webwork/from_openwebwork_dev_repository$ grep -R freeze *
webwork2/lib/WeBWorK/Utils.pm: return Storable::freeze( \@ordered_hash);
webwork2/lib/WeBWorK/AchievementEvaluator.pm:use Storable qw(freeze thaw);
webwork2/lib/WeBWorK/AchievementEvaluator.pm: #update counter, freeze localData and store
webwork2/lib/WeBWorK/AchievementEvaluator.pm: $userAchievement->frozen_hash(freeze($localData));
webwork2/lib/WeBWorK/AchievementEvaluator.pm: #freeze globalData and store
webwork2/lib/WeBWorK/AchievementEvaluator.pm: $globalUserAchievement->frozen_hash(freeze($globalData));
Thanks Nandor.
Arnie