I installed a fresh 9.1-RELEASE. I have no problem to install Perl 5.18. But on my old machine, it has a 9.1-STABLE kernel, I had perl5.16 installed. When I tried to install perl5.18. It always complains about a conflict with 5.16 no matter how many times I did make deinstall
of Perl 5.16.
I recompiled the kernel and rebuilt the world. I deleted all 5.14 and 5.6. Now it is 9.2-RELEASE. Now I am installing perl5.18. It's done. No complaint!