From perldoc -f perlvar

$^O The name of the operating system under which this copy of Perl was built, as determined during the configuration process. The value is identical to $Config{'osname'}. See also Config and the -V command-line switch documented in perlrun.
In Windows platforms, $^O is not very helpful: since it is always "MSWin32", it doesn't tell the difference between 95/98/ME/NT/2000/XP/CE/.NET. Use Win32::GetOSName() or Win32::GetOSVersion() (see Win32 and perlport) to distinguish between the variants.

Note that this might not be what you want , as it's not the runtime but the build-system of your perl-install.