Mail::Box - Modifying header w/ index value

I need to modify individual headers in a group with the same attribute name 
and have not been able to find a method to do so. My current goal is to 
rewrite To and CC headers (including refolding), but it is a general problem 
I need to solve (Received...). There is a well documented method to set() a 
header, but it doesn't take an index like get() does. I have also tried to 
get an array of fields and then modify the index I want, but there is no 
write API that I can find for Field objects. Finally, I tried a few 
combinations of deleting the field and using reset(), but I either ran into 
the documented issue caused by refcounts or ended with no change. I also 
reviewed Field::Fast code and could not find any code that changed the 
blessed array of private data ([1]) so what am I missing?

Thanks,
    Todd Richmond