I would offer that such testing is still reasonable, to ensure that the behavior does not change if/when some future version implements more complex wrapping around readline.

If the high-level specification says that the code behaves "like so" with/without a final newline, the testsuite should verify that, even if it is effectively verifying part of perl itself in the current implementation. If nothing else, this will blow up if some future perl changes that behavior, alerting to the need to either add code to maintain the previous behavior or revise the specification.