treewide: use perl.withPackages when possible

Since 03eaa48 added perl.withPackages, there is a canonical way to
create a perl interpreter from a list of libraries, for use in script
shebangs or generic build inputs. This method is declarative (what we
are doing is clear), produces short shebangs[1] and needs not to wrap
existing scripts.

Unfortunately there are a few exceptions that I've found:

  1. Scripts that are calling perl with the -T switch. This makes perl
  ignore PERL5LIB, which is what perl.withPackages is using to inform
  the interpreter of the library paths.

  2. Perl packages that depends on libraries in their own path. This
  is not possible because perl.withPackages works at build time. The
  workaround is to add `-I $out/${perl.libPrefix}` to the shebang.

In all other cases I propose to switch to perl.withPackages.

[1]: https://lwn.net/Articles/779997/

nixos/modules/system/activation/switch-to-configuration.pl

@@ -1,4 +1,4 @@
-#! @perl@
+#! @perl@/bin/perl
 
 use strict;
 use warnings;

nixos/modules/system/activation/top-level.nix

@@ -113,8 +113,7 @@ let
     configurationName = config.boot.loader.grub.configurationName;
 
     # Needed by switch-to-configuration.
-
-    perl = "${pkgs.perl}/bin/perl " + (concatMapStringsSep " " (lib: "-I${lib}/${pkgs.perl.libPrefix}") (with pkgs.perlPackages; [ FileSlurp NetDBus XMLParser XMLTwig ]));
+    perl = pkgs.perl.withPackages (p: with p; [ FileSlurp NetDBus XMLParser XMLTwig ]);
   };
 
   # Handle assertions and warnings

nixos/modules/system/boot/loader/grub/grub.nix

@@ -728,13 +728,17 @@ in
             utillinux = pkgs.util-linux;
             btrfsprogs = pkgs.btrfs-progs;
           };
+          perl = pkgs.perl.withPackages (p: with p; [
+            FileSlurp FileCopyRecursive
+            XMLLibXML XMLSAX XMLSAXBase
+            ListCompare JSON
+          ]);
         in pkgs.writeScript "install-grub.sh" (''
         #!${pkgs.runtimeShell}
         set -e
-        export PERL5LIB=${with pkgs.perlPackages; makePerlPath [ FileSlurp FileCopyRecursive XMLLibXML XMLSAX XMLSAXBase ListCompare JSON ]}
         ${optionalString cfg.enableCryptodisk "export GRUB_ENABLE_CRYPTODISK=y"}
       '' + flip concatMapStrings cfg.mirroredBoots (args: ''
-        ${pkgs.perl}/bin/perl ${install-grub-pl} ${grubConfig args} $@
+        ${perl}/bin/perl ${install-grub-pl} ${grubConfig args} $@
       '') + cfg.extraInstallCommands);
 
       system.build.grub = grub;

nixos/modules/system/etc/etc.nix

@@ -154,7 +154,7 @@ in
       ''
         # Set up the statically computed bits of /etc.
         echo "setting up /etc..."
-        ${pkgs.perl}/bin/perl -I${pkgs.perlPackages.FileSlurp}/${pkgs.perl.libPrefix} ${./setup-etc.pl} ${etc}/etc
+        ${pkgs.perl.withPackages (p: [ p.FileSlurp ])}/bin/perl ${./setup-etc.pl} ${etc}/etc
       '';
 
   };