- contains its normal length. "copy" magic receives the current
- element (i.e. the value) in $_[2]. $_[2] is also the current key in
- "fetch", "store", "exists" and "delete" callbacks.
+ contains its normal length. $_[2] is the current key in "copy",
+ "fetch", "store", "exists" and "delete" callbacks, although for
+ "copy" it may just be a copy of the actual key so it's useless to
+ (for example) cast magic on it. "copy" magic also receives the
+ current element (i.e. the value) in $_[3].