Quoting from CPAN's web site "CPAN is the Comprehensive Perl Archive Network, a large collection of Perl software and documentation." That really says it all. It is a central location where Perl developers can contribute the software they write.
CPAN has a means of standardizing installation, Makefile.pl (which is a Perl script which creates a Makefile with targets like "install", "test", "config", "clean", etc.). Makefile.pl typically uses the MakeMover module. It also has a means of registering a namespace for the module that a developer is contributing.
From the Boost web site "[Boost] provides free peer-reviewed portable C++ source libraries. The emphasis is on libraries which work well with the C++ Standard Library. One goal is to establish "existing practice" and provide reference implementations so that the Boost libraries are suitable for eventual standardization. Some of the libraries have already been proposed for inclusion in the C++ Standards Committee's upcoming C++ Standard Library Technical Report."
From what I can tell, unlike CPAN, Boost is a bit more focused on standards and review. That is, it is perhaps more Cathedral than Bazaar . Boost does not currently have a standard means of installation.
See Eric Raymond's essay The Cathedral and the Bazaar.