Did you write the test script before or after writing the module? Writing tests early accrues many benefits.
I think it's appropriate that you're using the same (perl) file permission techniques in the module and the script; that makes the test script more portable and easier to maintain than if you checked file permissions using Linux or Windows specific techniques, for example. After all, you're not testing perl itself, you're testing your module: you want to demonstrate, via your test script, that your module has a nice, clean, portable, easy to understand interface, and is easy to test in isolation. Testing file permissions in a non-perl way, while a good idea for testing perl itself, introduces unnecessary complexity and an unwelcome dependency for your module's test suite.