1. dumpbin.exe /exports the.dll
2. If you have a tool available to disassemble the dll, you can work out which calling convention was used. For the example above, the when built using _stdcall, the disassembly looks like:

   Disassembly of Function test  (0x00331000)
     SYM:test
     0x331000: PUSH     EBP                                  
     0x331001: MOV      EBP,ESP                              
     0x331003: MOVSX    EAX,DWORD PTR [EBP+0x8]              ; ARG:0x8
     0x331007: MOVSX    ECX,DWORD PTR [EBP+0xC]              ; ARG:0xC
     0x33100B: ADD      EAX,ECX                              
     0x33100D: MOVSX    ECX,DWORD PTR [EBP+0x10]             ; ARG:0x10
     0x331011: ADD      EAX,ECX                              
     0x331013: MOVSX    ECX,DWORD PTR [EBP+0x14]             ; ARG:0x14
     0x331017: ADD      EAX,ECX                              
     0x331019: POP      EBP                                  
     0x33101A: RET      0x10
   [download]

   When built with _cdecl it looks like

   Disassembly of Function test  (0x00331000)
     SYM:test
     0x331000: PUSH     EBP                                  
     0x331001: MOV      EBP,ESP                              
     0x331003: MOVSX    EAX,DWORD PTR [EBP+0x8]              ; ARG:0x8
     0x331007: MOVSX    ECX,DWORD PTR [EBP+0xC]              ; ARG:0xC
     0x33100B: ADD      EAX,ECX                              
     0x33100D: MOVSX    ECX,DWORD PTR [EBP+0x10]             ; ARG:0x10
     0x331011: ADD      EAX,ECX                              
     0x331013: MOVSX    ECX,DWORD PTR [EBP+0x14]             ; ARG:0x14
     0x331017: ADD      EAX,ECX                              
     0x331019: POP      EBP                                  
     0x33101A: RET
   [download]

   The significant part of that is the last (RET) line. In the former, the called code (the function itself) is responsible for cleaning up the stack, hence RET 0x10.

   In the latter case, the calling code is responsible for cleaning up the stack, hence the bare return.

3. Not with Win32::API that I am aware of.

   It might be possible to build a modified version to do it, but it would be messy. Using XS would be simpler and better.

4. I don't really understand this. Are you using W::API::Prototype::ApiLink() or Win32::API->new() and 'ssss'?

   If the latter, according to the docs, the 's' template does not mean 'short', but 'struct'.

   As mentioned above, shorts are placed on the stack as ints; the difference is in how they are addressed (16-bit indirect operands instead of 32-bit indirect etc.). I think you would have to use 'IIII' for the latter method.

5. I'm far from expert with XS.

   You would still need to determine which calling convention was used, but it should be possible to prototype the external call in the XS code to cater for either successfully, though you may need to do a little manual intervention if the dll was built with _cdecl.

If you are not getting traps when using 'IIII' (or ApiLink and 'SHORT'), then odds are that it was built using _stdcall and Win32::API may be all you need. Though if you are prepared to get stuck in with writing an XS wrapper, it would be considerably more efficient.