ModifyTo make Modules exclude !:: from its TCL setup, modify init.c as shown in this unified diff, and recompile.
@@ -703,6 +703,11 @@
        envsize += strlen( environ[i]) + 1;
+#ifdef __CYGWIN__
+       if( *environ[i] == '!')
+           continue;
+#endif
+
        /**
         **  Locate the equal sign and terminate the string at its position.
         **/