Advice for modifying a text file in real time while the game is running

[srjeffers]

Hello,

I'm new to modding in general so sorry if I ask anything that has an obvious answer.

I would like to slightly modify left 4 dead so that i can keep track of a few statistics within a text file (or possibly multiple text files). I would like to keep track of things such as zombie density, the player's health, and other factors. Would there be a way to write these types of things through modifying the text file while the game is being played? Would I need to code my own vpk file that would modify this or would something else need to be done? Any advice would be greatly appreciated.

Thanks for your time.