By looking at what your browser sends, potentially via Wireshark or maybe the Mozilla HTTP Live Headers extension. You can also dump what the script receives directly by not using CGI and printing everything that the script reads from STDIN to a file.

Most likely, your browser tells CGI that it is sending UTF-8 but isn't sending UTF-8. But without seeing the headers and data, that's always hard to tell.

Thank you. I have got my data, by removing use utf8::all; .