I'm not sure it really matters to most perl scripts what MTA is actually being used since most of them provide a /usr/local/bin/sendmail be it a symlink or not. If the program uses sockets or whatever the MTA will surely implement SMTP/ESMTP on port 25.

The MTA generally appears to work in the same way regardless with a few differences regarding headers and re-writing of messages... but I'm sure you'll configure whatever you change to so that it will not appear to have changed at all (as far as the users can see).

QMail seems like a good choice to me.

HTH - Mark