I would like it if cpanm gave notice of a successful verification.

It actually does, if cpanm --verify could find cpansign.bat, not just cpansign or cpansign.exe (I've reported this Win32 failure to App::cpanminus's repo). I patched my copy of cpanm to also search for the .bat ending when doing its which('cpansign') (and any other calls to which throughout), and now it does properly report a success:

c:\> cpansign --verify WWW::KeePassHttp
--> Working on WWW::KeePassHttp
Fetching http://www.cpan.org/authors/id/P/PE/PETERCJ/WWW-KeePassHttp-0
+.020.tar.gz ... OK
Fetching http://www.cpan.org/authors/id/P/PE/PETERCJ/CHECKSUMS ... OK
Verifying the SIGNATURE file ... Verified OK
Configuring WWW-KeePassHttp-0.020 ... OK
...
[download]

Since you are on linux or the linux-like Cygwin, I think cpanm --verify should work for you, as long as cpansign is in your $PATH.