-In the special case of C<len> magic and when the variable is an array, C<$_[2]> contains its normal length.
-C<$_[2]> is the current key in C<copy>, C<fetch>, C<store>, C<exists> and C<delete> callbacks, although for C<copy> it may just be a copy of the actual key so it's useless to (for example) cast magic on it.
-C<copy> magic also receives the current element (i.e. the value) in C<$_[3]>.
+Other arguments are specific to the magic hooked :
+
+=over 8
+
+=item -
+
+C<len>
+
+When the variable is an array, C<$_[2]> contains the normal length.
+The callback is also expected to return the new scalar or array length.
+
+=item -
+
+C<copy>
+
+C<$_[2]> is a either a copy or an alias of the current key, which means that it is useless to try to change or cast magic on it.
+C<$_[3]> is an alias to the current element (i.e. the value).
+
+=item -
+
+C<fetch>, C<store>, C<exists> and C<delete>
+
+C<$_[2]> is an alias to the current key.
+Nothing prevents you from changing it, but be aware that there lurk dangerous side effects.
+For example, it may righteously be readonly if the key was a bareword.
+You can get a copy instead by passing C<< copy_key => 1 >> to L</wizard>, which allows you to safely assign to C<$_[2]> in order to e.g. redirect the action to another key.
+This however has a little performance drawback because of the copy.
+
+=back