|Perl Monk, Perl Meditation
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?
Usually, you use make to do 3 things: build, test, and install. In the case where there's only .p[lm] files involved, there's probably no build necessary, but the makefile can still test and install.
After running perl Makefile.PL for a given pure Perl module, you might check out the resulting makefile to see what work it's doing for those abovementioned tasks.