I really think this has more to do with variable interpolation than it does with log4perl.

Comparing example A:

$self->{logger}->trace("total NbObsThis $i=$nbObservedThisNGram->[$i];
+ nbObsAll $i=$nbObservedAllNGrams->[$i]; total=$totalObservedAll; nbE
+xpected $i=$nbExpectedThis[$i]; chisq = $chiSquare[$i]");
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with example B:

$self->{logger}->trace('no interpolation');
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would be useful to demonstrate the difference in overhead between perl internals and log4perl. You're going to be stuck with interpolation performance no matter what logging framework you use.

I don't think so, I mean in the first place I got rid of this interpolation problem by testing the log level using a call to Log4perl:

if ($self->{logger}->is_trace()) { # for efficiency (not to compute th
+e expression) 
            $self->{logger}->trace("total NbObsThis $i=$nbObservedThis
+NGram->[$i]; nbObsAll $i=$nbObservedAllNGrams->[$i]; total=$totalObse
+rvedAll; nbExpected $i=$nbExpectedThis[$i]; chisq = $chiSquare[$i]")
}
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It helped indeed, but the cost of "is_trace()" was still prohibitive. Then I added a new variable $self->{disableLogger} set to true if the level is OFF (testing the log level only once) and replaced the condition with

if (!$self->{disableLogger} && $self->{logger}->is_trace()) { # test f
+or efficiency (not to compute the expression) {
            $self->{logger}->trace("total NbObsThis $i=$nbObservedThis
+NGram->[$i]; nbObsAll $i=$nbObservedAllNGrams->[$i]; total=$totalObse
+rvedAll; nbExpected $i=$nbExpectedThis[$i]; chisq = $chiSquare[$i]")
}
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and this takes way less time (according to NYTProf). As previously said, it seems that a simple call to is_debug() (no interpolation) implies a complex (and long) computation in Log4perl.

Better yet, use a constant instead of a hashref lookup, and the whole if block will be obliterated at compile time when you disable debugging.

use constant DEBUG => 0;
...
if (DEBUG) {
    # complex logging, which disappears from the op tree
}
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FWIW, my experience has been like Solo's, where you end up inadvertently evaluating the arguments to the logging methods, and this takes a lot of time (or a little bit of time in a tight loop that's called a zillion times).