You can use Win32API::File's createFile() [or CreateFile() if you are already into Win32 APIs or are just a masochist] to try to open the file without allowing "write" sharing. Such an attempt will fail if any other process currently has the file open for writing. Keep a list of "new" files and periodically try to open each this way. When you succeed, remove the file from the list and send it.

You may not want to keep the file open with no "write" sharing during the transfer. Although this sounds tempting as it ensures that you get a valid snap shot of the file contents and not half of the file contents from before some update and half from after that update, it also would cause other programs that attempt to write to the file to fail during that time (which will probably cause those programs to just give up).