Clarify that you need ActiveState perl 5.8 *or later* to build on Windows.

doc/src/sgml/install-windows.sgml

@@ -170,7 +170,7 @@ $ENV{PATH}=$ENV{PATH} . ';c:\some\where\bison\bin';
       ActiveState Perl is required to run the build generation scripts. MinGW
       or Cygwin Perl will not work. It must also be present in the PATH.
       Binaries can be downloaded from
-      <ulink url="http://www.activestate.com"></> (Note: version 5.8 is required,
+      <ulink url="http://www.activestate.com"></> (Note: version 5.8 or later is required,
       the free Standard Distribution is sufficient).
      </para></listitem>
     </varlistentry>