I wonder if the argument to use can simply be conditional, without needing a helper module?

use ($^O eq 'MSWIn32' ? 'MyAppWin32' : 'MyAppUnix');
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I think that should work in principle, since the comparison will be done before the use. It's just a matter of checking the docs to see what the differences are between barewords and normal strings in the use/require magic.

—John