From the perlvar documentation:

   $OSNAME
       $^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.      

Of course you could also just check and see which of the directories you are interested in exists, rather than worrying about what the operating system is.