Hmmm...I think the solution might not be that obvious. (especially once you start to worry about more complex constructs within the template, such as IFs etc).

Here's a sneaky thought - run the template, but give it a tied hash which traps and output the key on each attempted access to a hash value. You'll still miss any hash keys which are conditionally accessed, but it should do as a good first run.

Looks like Tie::Trace might do the job nicely.