78 would be ok, but WHY?
The usual argument says stuff about terminals not being able to handle characters in the last column correctly (then why the hell HAVE that column?), and 80-2 allowing the text to still be 80 wide after adding "> ", the quotation mark used in email.
However, I wouldn't even use such a terminal, and the quoting argument doesn't hold given replies to replies, when another "> " is added. And I indent all code (a habit I got from POD, and I like it) in mail anyway.
So I think 78 makes no sense at all, and this is one of the guidelines in Perl Best Practices that I choose to ignore. It's 80 for me.