You might try to get a slightly more meaningful error message like this:

open (MAIL, "|$mailProg") || die "Can't create pipe to $mailProg: $!";
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$! should contain the error message as a string and die will announce more about the location of the error without the "\n" on the end. The documentation for both open and perlvar talk about $!.

Also, if you felt like doing things in the exceedingly modern perl-cool way, you'd write it like this:

open my $mailer, "|-", $mail_prog or die "couldn't open pipe to $mail_
+prog: $!";
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Then $mailer will close itself when it goes out of scope and you don't pollute your package namespace with all-caps globs.