I see multiple ways to do this. If you don't care that you miss an update or two and just want to display the current state, you could use something like Cache::Memcached to always hold to current value in memcached. Then, your Tk could just poll whenever it feels like it wants to update.

Or, use a simple database (a minimal postgresql setup with a table or two will do nicely). This way, you don't have to solve the syncronisation problem but you also can keep as much historical records as you like. This also gives you a simple way to implement a sensors "replay" for debugging or to search through big datasets with minimum extra coding.

BREW /very/strong/coffee HTTP/1.1
Host: goodmorning.example.com

418 I'm a teapot