Revert "perl: Make 5.22 the default"

This reverts commit 7f547126d8. We're a
bit too close to release to make such fundamental changes.

pkgs/top-level/all-packages.nix

@@ -4908,7 +4908,8 @@ let
     fetchurl = fetchurlBoot;
   };
 
-  perl = perl522;
+  # Make perl522 the default once gnulib is updated to support it.
+  perl = perl520;
 
   php = php56;