perlPackages: add default meta.mainProgram (NixOS#176398)

Because perl packages are prefixed with the perl version, it means that
the `lib.getExe` heuristic will never point to the binary name. So we
provide the meta.mainProgram that overrides that, using the original
pname or parsed name. It's not perfect but should yield better results
already.

pkgs/development/perl-modules/generic/default.nix

@@ -35,6 +35,7 @@ lib.warnIf (attrs ? name) "builtPerlPackage: `name' (\"${attrs.name}\") is depre
 (let
   defaultMeta = {
     homepage = "https://metacpan.org/release/${lib.getName attrs}"; # TODO: phase-out `attrs.name`
+    mainProgram = attrs.pname or (builtins.parseDrvName attrs.name).name;
     platforms = perl.meta.platforms;
   };