A new number will be generated when roll is called. If you want to get a new number, then the value, of "A" for example, needs to be updated each time you want to get it. Once it's stored in a hash, it's stored in a hash. Referring to the variable in the code won't update the value, even if you put the call to roll("1d20") directly into the hash, as in:
%hash = (
A => roll("1d20"),
B => roll("1d20"),
C => roll("1d20"),
); # As soon as you get to this point in the code, the value is stored
+ (for good).
You need to figure out when you will want to store the value that was rolled, and when you will want to get a new value, and handle that accordingly. If the user wants to get a new "A" value, then you need to call roll again. If they want to get an old "A" value, then you need to refer to the value stored in the hash.
It seems like the point of the hash in your situation is to store things that don't dynamically change, but things that change as a result of the user doing something. However, if you want a roll to occur, you'll want a different mechanism for actually getting the roll, and the hash will be used just to store that roll.
I hope I am making sense.