I would use local for that:

{
    local $tempdir = '/tmp/foo';
    test_someting();
}
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Most likely you need to wrap all these test_something(...) function calls anyway with your code, and that's a good place to set up the localized test fixtures as well.

I'm wondering a bit how you plan to handle one such function calling another function. Should the fixtures be recreated or not?

sub test_someting2( $tempdir, $aValue ) {
    if( $^O !~ /mswin/i ) {
        test_someting("$tempdir/$aValue");
    } else {
        test_someting("$tempdir\\$aValue");
    };
}
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You would not call test functions by yourself, the system would do. In this specific case you'd use File::Spec anyway.

What is interesting in this case might be how to skip tests. I think the usual SKIP would work inside the test functions, but there might be some way to mark test functions to be skipped conditionally. Pytest uses "decorators" for this.

@pytest.mark.skipif(sys.platform != 'darwin', reason="Mac tests")
def test_func():
   ...
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