Disclaimer: I'm not sure about this, but aren't SSH ciphers negotiated automatically at the start of the session? Net::SFTP::Foreign just uses the ssh that's available on the system*, so as long as that's up to date, you should be able to test by just using the ssh command to connect to the server at the command line to test whether it works.

* Update: Ah, sorry, you're using backend => 'Net_SSH2'.