You can find the OS you are on with these special vars:

$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 the Config manpage
        and the -V command-line switch documented in the
        perlrun manpage.
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From the perldoc perlvar POD.

--xPhase