To Windows, these days, URL protocols are treated similar to file extensions. In fact, the mechanism to use can be the very same as just launching a file in its default program. You could use Win32::API with the ShellExecute API call, but just using the assistant command line tool program start, which comes with Windows, can be enough, and is much simpler to implement. Try this:

system('start', 'mailto://foo@bar.invalid');
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Works for me. And dead easy.