Why use perlbrew when you can use cpan/cpanm/cpanp directly?

Because the default perl usually comes with the system (at least on Unix systems). And quite often, the system depends on that perl version and configuration. Slackware is a rare exception: It comes with a quite recent perl version, but it does not depend on Perl. For other Linux distributions and other Unix systems, you better use the package manager (rpm, apt, whatever - RTFM) to install perl modules from system packages instead of installing from CPAN. If you want to use the latest versions of some perl module, better install an independent perl, e.g. using perlbrew or by compiling perl manually. This way, modules "too new" for the system won't interfere with the system perl.

Alexander