how about, instead of identifying the DLL as the library file, identify the LIB file instead

There's actually a nice little demo of accessing a Windows dll in 'perldoc Inline::C-Cookbook'. (Look for the section "Just Desserts".) And, yes, it links to user32.lib - though, admittedly, there's no explicit statement to tell you that.

(is) it possible to refer to multiple libraries in e.g. MYEXTLIB ?

You would just refer to the various .lib files in LIBS - and make sure that the dll's are in your path.

Cheers,
Rob