The References section has a link to an article that complements the documentation well. It says (on page 3):
Each appender can define a so-called appender threshold, a minimum level required for an oncoming message to be honored by the appender: 
    $appender->threshold($ERROR);

[download]
If the level doesn't meet the appender's threshold, then it is simply ignored by this appender.
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No matter how great and destructive your problems may seem now, remember, you've probably only seen the tip of them. [1]

In reply to Re^3: Log4Perl configuration problems by andreas1234567
in thread Log4Perl configuration problems by ribasushi

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