Thanks BrowserUK and ikegami for your efforts. I could not comprehend all infomations you gave me. But what I understood is that Win32::API is not suitable for using this dll. So I put this project aside.
The card reader is used via usb. It might have a built-in serial converter. If I find some more time I could try to access the raw data from the device. For which Linux (my preffered OS) could be useful.