+1, such convenience exports turn out into horrible debugging sessions costing devs lots of time.

I've been using a nixos config with these explicitly unset for a while now.

I'm not sure what your point is here, but it's pretty easy for libraries like this to end up in your system because of propagated inputs. I would personally prefer to go even further and remove /lib from the system profile altogether, however there are some practical reasons why we can't do that. That said I understand this might be convenient in some cases and it's possible to configure (for more than just perl packages) but it shouldn't be enabled by default IMHO.

The point is nix-env -i perlPackages.LWP and environment.systemPackages = [ pkgs.perlPackages.LWP ] should work and install the library for the user or system-wide.

For what it's worth, I don't think that these should work. Installing a Haskell library or an R package that way in Nix certainly won't work, and I don't see why it should be any different for Perl. The assumption that you can put a random library into your profile and henceforth all other software know that it's there and uses it automatically is false. We should not encourage users to assume that this would work.

Well, you can install perl + libraries system-wide. You just can't do it in the way you suggested above.