RT-Extension-MandatorySubject

I downloaded the extension from
http://search.cpan.org/dist/RT-Extension-MandatorySubject/ and followed
http://cpansearch.perl.org/src/ELACOUR/RT-Extension-MandatorySubject-0.03/README
step
by step;

This RT Extension should enforce users to fill the subject when creating a
ticket via the web interface. But, it did not work for me.

All these steps were successful
perl Makefile.PL
make
make test
make install

The following is added in RT_SiteConfig.pm
Set(@Plugins,(qw(RT::Extension::MandatorySubject)));

I also tried adding the above following way:
Set(@Plugins,qw(RT::Extension::MandatorySubject));

The server was restarted before testing.

I have read this extension is worked for many people. I am wondering what is
missing is my case?

Thank you in advance for your advise.
vm