That piece of doc expains dosish platforms, so it is closely related to Win32::GetOSVersion function.
perldoc Win32 contains an answer to your question:

  Win32::GetOSVersion()
    [CORE] Returns the array (STRING, MAJOR, MINOR, BUILD, ID), where
    the elements are, respectively: An arbitrary descriptive string, t
+he
    major version number of the operating system, the minor version
    number, the build number, and a digit indicating the actual
    operating system. For the ID, the values are 0 for Win32s, 1 for
    Windows 9X and 2 for Windows NT/2000/XP. In scalar context it
    returns just the ID.

    Currently known values for ID MAJOR and MINOR are as follows:

          OS                    ID    MAJOR   MINOR
          Win32s                 0      -       -
          Windows 95             1      4       0
          Windows 98             1      4      10
          Windows Me             1      4      90
          Windows NT 3.51        2      3      51
          Windows NT 4           2      4       0
          Windows 2000           2      5       0
          Windows XP             2      5       1
          Windows .NET Server    2      5       1

    Unfortunately as of June 2002 there is no way to distinguish betwe
+en
    .NET servers and XP servers without using additional modules.
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