Thanks Corion. I'm now using your approach and am very happy with it. I've changed $OS_TYPE to be 'Win32' and 'Unix' so I could replace:

if ($OS_TYPE eq 'win') {
  require My::Functions::Win32;
} else {
  require My::Functions::Unix;
};
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with:

eval "require My::Functions::$OS_TYPE";
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or:

require "My/Functions/$OS_TYPE.pm";
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Both seem to work ok. Is there a reason to prefer one over the other?