Actually, __declspec(dllexport) probably works fine. I had made the mistake of coding it as...

According to the (MS Docs), either of those is legal syntax. For their compiler at least. But see below.

But it exports it as XS_Foo_foo - which is not quite what I want.

That it is adding XS_* suggests to me that it is not a compiler facility as with MSC++, but rather a macro in one of the Perl specific header files somewhere. Perhaps something associated with the following from XSUB.H?

XSUB.h:#  define XS(name) __declspec(dllexport) XSPROTO(name)
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That said, The grep I just did that found that, also turned up the following tantalising snippet:

win32\win32.h:/* now even GCC supports __declspec() */
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The grep also turned up a bunch of other __declspec references in 5.10.0 header files. I haven't pursued any of them.

Seems like it is mostly there but not quite. Maybe what's needed is a Perl specific extended-decl-modifier-seq to add the XS decorations: say XSdllexport so as not to conflict with the 'standard' dllexport.