Assuming that the Postgres source tree is rooted at $PGSRC, then the Pl/perl source code is located in $PGSRC/src/pl/plperl.
To build, simply do the following:
cd $PGSRC/src/pl/plperl perl Makefile.PL make
This will create a shared library file. On a Linux system, it will be named plperl.so. The extension may differ on other systems.
The shared library should then be copied into the lib subdirectory of the postgres installation.
The createlang command is used to install the language into a database. If it is installed into template1, all future databases will have the language installed automatically.