t/01-callback.t is incomplete

[haarg]

The test script t/01-callback.t tries to pass a callback to overload::open->import, but overload::open does not have an import method. So the callback is never used for anything, and the test is not testing anything that it intended to.

Perl will ignore an import call if there is no import method. Future versions of perl are intending to make it an error to pass arguments to an undefined import method, which will cause the test to fail.