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";
[download]

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

$num = increment($num);

print "$num\n";
[download]

I don't think my sub that uses references is any better than the one that doesn't. It just shows two ways of doing the same thing.

Because you pass $num as a reference, the subrutine can change num. Then you can use the function's return for something else.

Update: chas! I've fixed the example to use ++$arg rather than $arg++.

AND! Woah. Baked my noodle with that one. So the only reason I've been convinced that subrutine args were passed as values was because I was using shift all the time!? Man... aye aye aye...