[staging] perl: 5.38.2 -> 5.40.0 (#333286)

After discussing on Matrix in the [#staging](https://matrix.to/#/#staging:nixos.org) room, I'm merging this as-is without waiting for changes to the `enableParallelBuilding = false;` line.

There are several in-the-wings PRs (one from @emilazy) that attempt to bring load-limit, which is a more sensible accounting of the work that a derivation is doing, into Nix itself. If and when that lands, we'll be able to just set `enableParallelBuilding = true;` again, without any specific casing around the `NIX_BUILD_CORES` stdenv variable.

nixos/tests/all-tests.nix

@@ -1055,6 +1055,7 @@ in {
   unbound = handleTest ./unbound.nix {};
   unifi = handleTest ./unifi.nix {};
   unit-php = handleTest ./web-servers/unit-php.nix {};
+  unit-perl = handleTest ./web-servers/unit-perl.nix {};
   upnp.iptables = handleTest ./upnp.nix { useNftables = false; };
   upnp.nftables = handleTest ./upnp.nix { useNftables = true; };
   uptermd = handleTest ./uptermd.nix {};

nixos/tests/web-servers/unit-perl.nix

@@ -0,0 +1,46 @@
+import ../make-test-python.nix (
+  { pkgs, ... }:
+  let
+    testdir = pkgs.writeTextDir "www/app.psgi" ''
+      my $app = sub {
+          return [
+              "200",
+              [ "Content-Type" => "text/plain" ],
+              [ "Hello, Perl on Unit!" ],
+          ];
+      };
+    '';
+
+  in
+  {
+    name = "unit-perl-test";
+    meta.maintainers = with pkgs.lib.maintainers; [ sgo ];
+
+    nodes.machine =
+      {
+        config,
+        lib,
+        pkgs,
+        ...
+      }:
+      {
+        services.unit = {
+          enable = true;
+          config = pkgs.lib.strings.toJSON {
+            listeners."*:8080".application = "perl";
+            applications.perl = {
+              type = "perl";
+              script = "${testdir}/www/app.psgi";
+            };
+          };
+        };
+      };
+    testScript = ''
+      machine.wait_for_unit("unit.service")
+      machine.wait_for_open_port(8080)
+
+      response = machine.succeed("curl -f -vvv -s http://127.0.0.1:8080/")
+      assert "Hello, Perl on Unit!" in response, "Hello world"
+    '';
+  }
+)