I'm pretty sure that you need to change how_many($beer--) to how_many(--$beer). After you take one down, and pass it around, unless your vision is blurrey, there's one fewer beer.

For the benefit of those that don't know, $var-- returns the value of $var, then decrements it. --$var first decrements, then returns $var

If the eval engine in my brain is correct, --$var++ will return $var - 1, but the value of $var will still be the same ;)