how to use the function of "feature selection" under AI::Categorizer

Hello,

I¡¯d like to select more important features using AI::Categorizer, and so
modified demo.pl as follows
=== FROM === 
my $k = AI::Categorizer::KnowledgeSet->new( verbose => 1 );
=== TO ===
my $k = AI::Categorizer::KnowledgeSet->new( verbose => 1,
   feature_selector => new AI::Categorizer::FeatureSelector::DocFrequency(
¡¡   verbose => 1,
¡¡   features_kept => 1000
¡¡ )
);
=== END ===
I observed the performance according to change the value of features_kept,
but the performance is always same. I¡¯d appreciate it if you tell me how
to do the feature selection using AI::Categorizer? 

Thank you very much in advance.

Jae-Hoon.