I gave this a test with the exact code you gave on my machine, and it worked great!
$ perl test.pl
( 0 , 0 ) => this
( 1 , 1 ) => is
( 2 , 2 ) => a
( 3 , 3 ) => test
( 4 , 1 ) => 什麼
(PM may convert the text (traditional Chinese "shenme" -- what) into an entity here, but it definitely worked in my xterm)
It may be that whatever you're using to view the file isn't expecting UTF-8; or, perhaps the encoding in the XLSX itself isn't UTF-8 (but I'm not sure if that's an option in XLSX files or what!).