Using sftp would mean the senders would have to change their methods (but not much), although in some cases they'd be replacing their client software entirely.
Alternatively they could keep using good ole' ftp - if you were to run it through OpenSSH using portforwarding. Clients will just change their destination FTP server to a local one, and pushing to them should be pretty easy with a couple tweaks.
Since the encryption in both SSL and SSH is most always coming from the same cryptography toolkit (usually openSSL), and either type/level of encryption from these packages is probably more than good enough to protect your data against prying eyes, it comes down to deciding what production methods are easier to migrate into, with the least hassle for your clients.
PS- why bother writing new Perl code unless you need additional functionality you didn't yet mention, unless of course, you've been reading the Monk quips and were inspired by the one that says "Go ahead - be a heretic" :)