README.PERL: clarify "matching" Perl requirement on Windows.

 Notes on Perl on Windows

 ------------------------

 If you will build on Cygwin (and possibly some other POSIX layers),

 Perl is already part of your distribution.  Simply use the Cygwin

 package manager to make sure Perl gets installed.

 Otherwise, you will need to install Perl separately.  The Perl

 package that we know of is ActiveState Perl, available from

 There are a number of build targets that can be viewed as "Windows".

 Indeed, there are VC-* configs targeting VisualStudio C, as well as

 MinGW and Cygwin. The key recommendation is to use "matching" Perl,

 one that matches build environment. For example, if you will build

 on Cygwin be sure to use the Cygwin package manager to install Perl.

 For VC-* builds we recommend ActiveState Perl, available from

 http://www.activestate.com/ActivePerl.

 Notes on Perl on VMS