I thought knew what make and Makefiles do for C and C++ projects

I'm confused... why would you think they'd be any different in relation to module distributions? It's exactly the same function.

Normally CPAN modules come with a Makefile.PL file (or more recently Build.PL, but that's another topic) which you run the Perl interpreter against to create a Makefile which then allows you to run:

$ make
   {spew gets issued}
$ make test 
   {test result spew here}
$ make install 
   {installation spew goes here}
[download]

Now, I've made a huge assumption here that you're on some flavor of *nix which I realize is a bad assumption. If you're on some flavor of *dows YMMV.

If I get a CPAN module which only contains one .pl file, why do I need a Makefile along with it? Why wouldn't it be enough just installing the .pl file in a directory, and making sure that directory is in @INC?

That didn't quite make sense to me. The CPAN modules I've seen come with (as I said above) Makefile.PL files and/or Build.PL files. Either way if you run Perl against the Makefile.PL file it will "do the right thing®" while building the Makefile for the particular system you are on and the way the Perl interpreter on your system was built.