rsnapshot: fix build by using older perl

pkgs/top-level/all-packages.nix

@@ -2256,6 +2256,7 @@ let
   rng_tools = callPackage ../tools/security/rng-tools { };
 
   rsnapshot = callPackage ../tools/backup/rsnapshot {
+    perl = perl516; # fails to create docs: POD document had syntax errors
     # For the `logger' command, we can use either `utillinux' or
     # GNU Inetutils.  The latter is more portable.
     logger = inetutils;