This is the revision history for why not system perl in <freenode.net:##NULL>

factoid_idsubjectcopulapredicateauthornamespace
11294 why not system perl is 1) You can cause system libraries to find newer versions of modules than they were expecting, 2) If the package manager upgrades perl, your CPAN-installed modules will be broken, 3) On perls older than 5.12, updates to dual-life modules must overwrite core modules or they will appear not to update due to @INC order grinnz <*:##NULL>
11286 why not system perl is 1) You can cause system libraries to find newer versions of modules than they were expecting, 2) Sometimes the newer module will appear not to update due to @INC order, 3) On older perls you might actually overwrite files managed by the package manager, 4) If the package manager upgrades perl, your CPAN-installed modules will be broken grinnz_ <*:##NULL>
11285 why not system perl is 1) You can cause system libraries depending on the packaged perl to find newer versions of modules than they were expecting, 2) Sometimes the newer module will still be later in @INC than core modules so they will appear not to update, 3) On older perls you might actually overwrite files managed by the package manager grinnz_ <*:##NULL>