I have a number of web bots I made with WWW::Mechanize and your code looks fine for me.
When using message boards, there's a few things to keep in mind.
Some boards have reload times- if you load the page and post the data too quickly, the bot protection will catch you and prevent your last action. Use a 5 or 10 second sleep() time between actions to bypass the bot traps and to be nicer to the server's resources
Some forums do use redirects once you sign in or after something is posted. Some forums also require the very next page to load before it'll post your data. If your forum redirects you upon successful login but does not say you logged in, chances are you weren't redirected to the page that set your SSID or cookie. Go through it manually and see where you're being redirected, then load the page using WWW::Mechanize AFTER your last action. I'm 95% certain this is your current problem.
"Age is nothing more than an inaccurate number bestowed upon us at birth as just another means for others to judge and classify us"