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]")
}
[download]

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]")
}
[download]

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.