I suspect you have a typo in the name. The sit(e) is a typo. Have a look in the directory:

C:\perl\site\lib\WWW\
[download]

Do you see Mechanize.pm there? If so you have a typo in your use statement, if not you have not installed it because this is where it ends up. BTW you don't need to extract into C:\perl\bin\ to do an install. See A Guide To Installing Modules