+ $_[-1] is the "B::OP" object for the current op.
+
+ Both result in a small performance hit, but just getting the name is
+ lighter than getting the op object.
+
+ These callbacks are executed in scalar context and are expected to
+ return an integer, which is then passed straight to the perl magic
+ API. However, only the return value of the *len* magic callback
+ currently holds a meaning.
+
+ Each callback can be specified as :
+
+ * a code reference, which will be called as a subroutine.
+
+ * 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.
+
+ * a reference to "undef", in which case a no-op magic callback is
+ installed instead of the default one. This may especially be helpful
+ for *local* magic, where an empty callback prevents magic from being
+ copied during localization.
+
+ Note that *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 :