[GENERAL] Building PL/Perl procedural language without --with-perl configure flag

I am trying to build PL/Perl procedural language to my PostgreSQL server installation but I want to do it without passing --with-perl flag to the configure script of PostgreSQL. It would also help if someone can explain the meaning of --with-perl flag in a detailed fashion altogether as it might help me determine the actual process of installing PL/Perl. There are two parts to the question however how do I get the source of PL/Perl firstly and how do I build it against an already built version of PostgreSQL as in when I have the (bin, lib, share) for PostgreSQL and what all pre-requisites are needed to install PL/Perl in the first place?

Thanks