It really depends where the text you want to extract is located. If the text is in the window title, then you need to call XGetWMName and/or XGetProperty(NET_WM_NAME) ( the latter is available in X11::Protocol , possibly the former also).

If the text is in some selectable text areas, selecting these with mouse will get the text copied in the X clipboard, which can then be accessed again by X11 core calls (XConvertSelection/XGetWindowProperty). Note that X clipboard is more complicated than win32 clipboard because you also need to track X11 events.

Finally, if none of these, I don't know if there's any generic way to query text from arbitrary application, and your best bet would be indeed, as suggested, to use OCR. Take a look at OCR::Naive that does exactly this.