For parameter errors, I prefer to throw an exception (well, a croak, which is as close to an exception as Perl gets). It prevents the caller from making stupid mistakes (like not passing that parameter at all), but still allows them to wrap it in an eval{} if they need to gracefully handle edge cases where that parameter wasn't set for some reason. (Even in those cases, they probably want to know about the error somehow, because it's likely a bug. This parameter is mandatory, so presumably we can't do anything without it).