thesa's advice is basically correct, but your point iso-8859-1 being the "default" encoding for many apps is also relevant. It's true that more and more apps (esp. browsers and browser-based email clients) are making it easier to change character encoding at the display as needed, but it's also true that a lot of them (and the people who use them) still treat the legacy iso-8859 and cp12.. as the "default".

If you would prefer your email output to be iso-8859-1, use the Encode module on the text data, or PerlIO control on the output file handle, in order to assure that the text is written with that encoding; assuming the email text is going out via a file handle the easiest thing would probably be:

binmode $fh, ":encoding(iso-8859-1)";  # convert internal utf8 to lati
+n1 on output
[download]

If you do this, you should still heed thesa's advice, and explicitly declare what character set you're using in the MIME header.