You can find out which operating system you are running on by looking at the $^O variable (see perlvar for details).

The easiest way to conditionally "use" modules is to replace the "use" with calls to "require" and "import" - as these are executed at runtime.

if ($some_condition) {
  require SomeModule;
  SomeModule->import;
} else {
  require SomeOtherModule;
  SomeOtherModule->import;
}
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Alternatively, you could look at if.