mod_rewrite is an rule-based script-like language implemented as an apache module. very easy to do what foe asked.
You can very sophisticated conditioning not only upon referer.
I'd suggest to read the mod_rewrite tutorial before deciding to do something on script. you will be surprised what you can achieve with mod_rewrite.
mod_rewrite is a little complex to grasp and master - but once you are "there" you will be a happy webMaster
