I think the die-message is available in $@ only in case the die happens within an eval{} block (in which case I personally wouldn't want to be mailed).

If the die happens outside of an eval, however (i.e. when the script atually dies), the message is only available in @_.   (Also, I suppose your code should be mail($err), as the localized $@ would always be empty...)

In other words, I'd use something like

#!/usr/bin/perl

use strict;
use warnings;
use Carp;

$SIG{__DIE__} = sub {
    die @_ if $^S;   # skip handler for eval{} blocks
    mail(@_);
};

sub mail {
    chomp(my $msg = shift);
    print qq(mailing "$msg"...\n\n);
}

sub test {
    eval { croak "bar"; };  # doesn't send mail
    croak "foo";            # sends mail (and dies)
}

test();

__END__
$ ./827866.pl
mailing "foo at ./827866.pl line 19
        main::test() called at ./827866.pl line 22"...

foo at ./827866.pl line 19
        main::test() called at ./827866.pl line 22
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