diff --git a/Library/Homebrew/brew.sh b/Library/Homebrew/brew.sh
index 56f2a5d58a..ca162a051c 100644
--- a/Library/Homebrew/brew.sh
+++ b/Library/Homebrew/brew.sh
@@ -1102,6 +1102,9 @@ then
 fi
 unset SUDO
 
+# Remove internal variables
+unset HOMEBREW_INTERNAL_ALLOW_PACKAGES_FROM_PATHS
+
 if [[ -n "${HOMEBREW_BASH_COMMAND}" ]]
 then
   # source rather than executing directly to ensure the entire file is read into
diff --git a/Library/Homebrew/build.rb b/Library/Homebrew/build.rb
index de47babe8c..0cfdd8d5e6 100644
--- a/Library/Homebrew/build.rb
+++ b/Library/Homebrew/build.rb
@@ -227,7 +227,10 @@ class Build
 end
 
 begin
-  ENV.delete("HOMEBREW_FORBID_PACKAGES_FROM_PATHS")
+  # Undocumented opt-out for internal use.
+  # We need to allow formulae from paths here due to how we pass them through.
+  ENV["HOMEBREW_INTERNAL_ALLOW_PACKAGES_FROM_PATHS"] = "1"
+
   args = Homebrew::Cmd::InstallCmd.new.args
   Context.current = args.context
 
diff --git a/Library/Homebrew/env_config.rb b/Library/Homebrew/env_config.rb
index 74679b02fd..df1ea287a7 100644
--- a/Library/Homebrew/env_config.rb
+++ b/Library/Homebrew/env_config.rb
@@ -632,6 +632,9 @@ module Homebrew
 
     sig { returns(T::Boolean) }
     def forbid_packages_from_paths?
+      # Undocumented opt-out for internal use.
+      return false if ENV["HOMEBREW_INTERNAL_ALLOW_PACKAGES_FROM_PATHS"].present?
+
       return true if ENV["HOMEBREW_FORBID_PACKAGES_FROM_PATHS"].present?
 
       # Provide an opt-out for tests and developers.
diff --git a/Library/Homebrew/formulary.rb b/Library/Homebrew/formulary.rb
index 1d30222bd5..92570e4e03 100644
--- a/Library/Homebrew/formulary.rb
+++ b/Library/Homebrew/formulary.rb
@@ -619,17 +619,21 @@ module Formulary
       if Homebrew::EnvConfig.forbid_packages_from_paths?
         path_realpath = path.realpath.to_s
         path_string = path.to_s
-        if !path_realpath.start_with?("#{HOMEBREW_CELLAR}/", "#{HOMEBREW_LIBRARY}/Taps/", "#{HOMEBREW_CACHE}/") &&
-           (path_string.include?("/") || path_string.end_with?(".rb"))
-          raise <<~WARNING
-            Rejecting formula at #{path_string} because it's not in a tap.
-                   Homebrew requires formulae to be in a tap.
+        unless path_realpath.start_with?("#{HOMEBREW_CELLAR}/", "#{HOMEBREW_LIBRARY}/Taps/", "#{HOMEBREW_CACHE}/")
+          if path_string.include?("./") || path_string.end_with?(".rb") || path_string.count("/") != 2
+            raise <<~WARNING
+              Rejecting formula at #{path_string} because it's not in a tap.
+                    Homebrew requires formulae to be in a tap.
 
-            To create a tap, run e.g.
-              brew tap-new <user|org>/<repository>
-            To create a formula in a tap run e.g.
-              brew create <url> --tap=<user|org>/<repository>
-          WARNING
+              To create a tap, run e.g.
+                brew tap-new <user|org>/<repository>
+              To create a formula in a tap run e.g.
+                brew create <url> --tap=<user|org>/<repository>
+            WARNING
+          elsif path_string.count("/") == 2
+            # Looks like a tap, let's quietly return but not error.
+            return
+          end
         end
       end
 
diff --git a/Library/Homebrew/postinstall.rb b/Library/Homebrew/postinstall.rb
index cde5b8b676..f69d270de3 100644
--- a/Library/Homebrew/postinstall.rb
+++ b/Library/Homebrew/postinstall.rb
@@ -14,7 +14,10 @@ require "cmd/postinstall"
 require "json/add/exception"
 
 begin
-  ENV.delete("HOMEBREW_FORBID_PACKAGES_FROM_PATHS")
+  # Undocumented opt-out for internal use.
+  # We need to allow formulae from paths here due to how we pass them through.
+  ENV["HOMEBREW_INTERNAL_ALLOW_PACKAGES_FROM_PATHS"] = "1"
+
   args = Homebrew::Cmd::Postinstall.new.args
   error_pipe = Utils::UNIXSocketExt.open(ENV.fetch("HOMEBREW_ERROR_PIPE"), &:recv_io)
   error_pipe.fcntl(Fcntl::F_SETFD, Fcntl::FD_CLOEXEC)
@@ -29,7 +32,7 @@ begin
   formula.run_post_install
 # Handle all possible exceptions.
 rescue Exception => e # rubocop:disable Lint/RescueException
-  error_pipe.puts e.to_json
-  error_pipe.close
+  error_pipe&.puts e.to_json
+  error_pipe&.close
   exit! 1
 end
diff --git a/Library/Homebrew/test.rb b/Library/Homebrew/test.rb
index 338e4d7d9b..1f2b1c8956 100644
--- a/Library/Homebrew/test.rb
+++ b/Library/Homebrew/test.rb
@@ -19,7 +19,10 @@ require "json/add/exception"
 DEFAULT_TEST_TIMEOUT_SECONDS = 5 * 60
 
 begin
-  ENV.delete("HOMEBREW_FORBID_PACKAGES_FROM_PATHS")
+  # Undocumented opt-out for internal use.
+  # We need to allow formulae from paths here due to how we pass them through.
+  ENV["HOMEBREW_INTERNAL_ALLOW_PACKAGES_FROM_PATHS"] = "1"
+
   args = Homebrew::DevCmd::Test.new.args
   Context.current = args.context
 
@@ -55,8 +58,8 @@ begin
   end
 # Any exceptions during the test run are reported.
 rescue Exception => e # rubocop:disable Lint/RescueException
-  error_pipe.puts e.to_json
-  error_pipe.close
+  error_pipe&.puts e.to_json
+  error_pipe&.close
 ensure
   pid = Process.pid.to_s
   if which("pgrep") && which("pkill") && system("pgrep", "-P", pid, out: File::NULL)