The original tarfile is created on a linux box using
tar on a group of files that are variously text, and
gzip'd binary. It really is just a regular tar file.

I then do a gpg --clearsign <filename> and get a
filename.asc as output.

I thought the --clearsign option would only sandwich the
file between 2 text strings, but that looks like a
non-starter. Even turning off compression, -z 0,
doesn't seem to do it. So I've gone back to trying
to get GnuPG::Interface to decrypt a .gpg signed file
output by gpg --sign <filename>.

There's something about GnuPG::Interface I'm not
getting ...