Whether your sub actually returns anything to the caller is one thing, but whether its return value means anything is another.

In the module's documentation, it's fine to either leave out a mention of what the sub returns, or explicitly say that what the sub returns is unspecified. This tells anybody using your sub that they should not rely on anything it returns. This way, in a later version of the module, if you decide that you want to return true, or a status code, or some kind of error message, you can, and you won't break any existing code.

For a little more on this, see the "Specifying semantics" section of Ned Batchelder's essay on interfaces. He uses the term "undefined" instead of "unspecified", but that's just because the languages he usually programs in probably don't have undef.