Merge nixpkgs.config.perlPackageOverrides

nixos/modules/misc/nixpkgs.nix

···
       packageOverrides = pkgs:
         optCall lhs.packageOverrides pkgs //
         optCall (attrByPath ["packageOverrides"] ({}) rhs) pkgs;
+    } //
+    optionalAttrs (lhs ? perlPackageOverrides) {
+      perlPackageOverrides = pkgs:
+        optCall lhs.perlPackageOverrides pkgs //
+        optCall (attrByPath ["perlPackageOverrides"] ({}) rhs) pkgs;
     };
 
   configType = mkOptionType {