Firstly, thanks for developing this as open source software and secondly thanks for thinking about releasing it via CPAN. Thirdly, thanks for asking about doing so first. :-)

Should I prepare to upload under the App: namespace upload under LSeismic_Unix-0.7_6 or is this mixed-language project not appropriate for uploading to CPAN.

Personally, I think mixed-language projects are fine for uploading to CPAN. If your code is more of an app than a module then, yes, as release under App::* would perhaps be more appropriate. Note that otherwise your preferred name would mean creating a new top-level namespace in CPAN and that really is frowned upon unless you have a very good reason for doing so. Also, I would suggest you use the N.NN_NN version naming system, ie. 0.07_06 in this case as that is what both users and the toolchain components will most likely expect.

I have chosen the name carefully and a quick google search shows that it can be directly related to a C-based open-source application (Seismic Unix) that it needs to function. ("L" stands for Louisiana).

Would anyone know that L stands for Louisiana? Would anyone know what Louisiana means in the context of this module?

PAUSE suggested I contact PrePAN users for advice but I have not been able to enter their site effectively.

That is a concern. I see no new modules added to PrePAN in the last 11 months. As a github user you should have easy access. If you can explain what is preventing you from posting there we might be able to help with that (or suggest who could).

Update: It is now the 15th of September 2022 and the PrePAN has gone. The domain has been taken by someone else and is now unrelated to Perl. :-(