I have a script that sends mail by calling sendmail directly like in your example. Several of our customers are now using qmail with the sendmail emulation and the script works fine for them, sending various types of mime and plain text messages.

The only problem we ran into with our first customer who tried the script with qmail was that it's pickier than sendmail about getting the end of line characters correct, whereas sendmail will take incorrectly formatted input without complaining. But if you use a module like Mime::Lite to generate the message I imagine it will do that correctly for you. (We haven't completely started relying on modules for email stuff just because it's usually easier and quicker to fix a couple of lines in our old code than to rewrite it to use modules to generate the mails.)