This article was published on perl.com in September 2002. Here's the latest errata list:
The article says on page 3:
Each appender carries an additivity flag. If this is set to a false value, like in $appender->additivity(0); then the message won't bubble up further in the hierarchy after the appender is finished.
This is incorrect, appenders don't carry an additivity flag, only
loggers do. Please replace appender in the above
paragrph by logger and it makes sense.
Thanks to Dave Weber for pointing this out.
Thanks to Jon Bjornstad for pointing this out.