If you look at perlvar under the section for $^O, there's a link to a section of the perlport documentation that has a detailed listing of the OS strings returned by $^O.

Also, the perlvar documentation for the variable points out that $^O is not too useful in distinguishing exactly which version of Windows the code is running on (XP, Vista, etc.) since it always returns MSWin32. There's a note about using some functions from the Win32 module to get more detailed information about the version of Windows.

Just wanted to toss out that information in case others might find it useful.