Assuming you're asking me, it's because I've been using Perl from way before there was a perlbrew, and have not made the leap. Heck, I only recently started using cpanm to install modules. In order to make effective use of perlbrew, I need to become more conversant with managing my installed modules in the perlbrew world, and I probably need to do some disk space rearranging given how full my system disk is these days.