Since POSIX is core, you should already have it. Since you apparently do not, you should reinstall your perl (cause chances are more things are missing), otherwise

perldoc perltoc |grep -i install
File STDIN:
            ExtUtils::Install, ExtUtils::Installed, ExtUtils::Liblist,
  perlmodinstall - Installing CPAN Modules
            module (sometimes unnecessary), INSTALL the module
        How do I install a module from CPAN?

Tutorials -> A Guide to Installing Modules

Now, POSIX does not come as a CPAN distribution per se (it's CORE), but it's still in the right format -> http://search.cpan.org/src/TIMB/perl5.004_02/ext/POSIX/

MJD says "you can't just make shit up and expect the computer to know what you mean, retardo!"

I run a Win32 PPM repository for perl 5.6.x and 5.8.x -- I take requests (README).

** The third rule of perl club is a statement of fact: pod is sexy.