The $^O builtin variable will tell you.

From perldoc 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 the Config manpage
         and the -V command-line switch documented in the
         perlrun manpage.
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--
Mike