Well ... not quite. The require will die ... so you need eval. Also, perl treats barewords special here - if you use a string, you'll need it to be 'Win32/Service.pm'. Stick to the barewords when possible ;-)

$runningOn = 'unix';
eval {
  require Win32::Service;
  $runningOn = 'windows';
};
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Of course, we assume that Win32::Service never fails to load, but that's ok, since we make that assumption about most modules.