The name of the operating system under which this copy of Perl was built, as determined during the configuration process. For examples see "PLATFORMS" in perlport.
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.
This variable was added in Perl 5.003.