Tilde escape commands in mailx for instances. Data input needs to be accurately checked to avoid abuses. That's the reason to avoid in generalthe use of such a command. Moreoveri, perl has different proper and powerful methods to send e-mail. There is no need to spawn a programlike that. So, that's also a stylistic issue.
I could not trick mailx (at least the debian version) to do something nasty by command execution if it detects non-interactive mode. I've seen in the sources that it checks on the basis of isatty() if it's in interactive mode and that looks quite stable. So I don't think this is a big issue.
Willi