sub increment {
    my $arg_ref = shift;
    $$arg_ref++;
    if (is_bad($$arg_ref)) {
        return "FAILURE!";
    } 
    return undef;
}

my $num = 0;
my $err;

$err = increment(\$num);
if ($err) {
    die $err;
}

print "$num\n";

##</code><code>##

sub increment {
    my $arg = shift
    if (is_bad($arg)) {
        die "FAILURE";
    }
    return ++$arg;
}
my $num = 0;

$num = increment($num);

print "$num\n";