+Each callback can be specified as :
+
+=over 4
+
+=item *
+
+a code reference, which will be called as a subroutine.
+
+=item *
+
+a string reference, where the string denotes which subroutine is to be called when magic is triggered.
+If the subroutine name is not fully qualified, then the current package at the time the magic is invoked will be used instead.
+
+=item *
+
+a reference to C<undef>, in which case a no-op magic callback is installed instead of the default one.
+This may especially be helpful for I<local> magic, where an empty callback prevents magic from being copied during localization.
+
+=back
+
+Note that I<free> magic is never called during global destruction, as there is no way to ensure that the wizard object and the callback were not destroyed before the variable.
+
+Here is a simple usage example :