RT-Extension installation issue

I am trying to install RT-Extension-MandatorySubject for RT 5.0.2 installation.
I followed the exact steps yet the first step " perl Makefile.PL " in the documentation gave me the following issue:

(rt5venv) root@devrt5:/opt/rt5/local/plugins/RT-Extension-MandatorySubject-0.04# perl Makefile.PL

include /opt/rt5/local/plugins/RT-Extension-MandatorySubject-0.04/inc/Module/Install.pm

Unknown function is found at Makefile.PL line 3.

Execution of Makefile.PL aborted due to runtime errors.

Inside the Makefile.PL in line 3, I have this line: RTx(‘RT-Extension-MandatorySubject’);