diff --git a/pkgs/build-support/mono-dll-fixer/builder.sh b/pkgs/build-support/mono-dll-fixer/builder.sh
deleted file mode 100644
index 67abc465a9b..00000000000
--- a/pkgs/build-support/mono-dll-fixer/builder.sh
+++ /dev/null
@@ -1,4 +0,0 @@
-source $stdenv/setup
-
-substitute $dllFixer $out --subst-var-by perl $perl/bin/perl
-chmod +x $out
diff --git a/pkgs/build-support/mono-dll-fixer/default.nix b/pkgs/build-support/mono-dll-fixer/default.nix
index 8f7d1e795d7..09a986015ee 100644
--- a/pkgs/build-support/mono-dll-fixer/default.nix
+++ b/pkgs/build-support/mono-dll-fixer/default.nix
@@ -1,8 +1,11 @@
 {stdenv, perl}:
-
 stdenv.mkDerivation {
   name = "mono-dll-fixer";
-  builder = ./builder.sh;
   dllFixer = ./dll-fixer.pl;
+  dontUnpack = true;
+  installPhase = ''
+    substitute $dllFixer $out --subst-var-by perl $perl/bin/perl
+    chmod +x $out
+  '';
   inherit perl;
 }