Do you know why a specific behavior is not defined? Just curious.

The behaviour is actually quite predictable, but it's probably not one the Perl developers want to be held to. If that's the case, the warning in the documentation serves to avoid a dependancy on the current behaviour.

In case you're curious,

The current compile-time behaviour of my is to define the variable for the rest of the scope, starting with the next statement.

The current run-time behaviour of my is to place an instruction on the stack to clear (if refcount = 0) or replace (if refcount > 0) the variable on scope exit. It also returns the variable as an lvalue.