My pencil and paper One Time Pad works fine without modular math ...... or does it?
If you are not using modular arithmetic and an eavesdropper observes the ciphertext 64, they immediately know that the plaintext is 39 (encrypted with the key 25). With modular arithmetic, if the ciphertext 39 is observed, the plaintext can range from 14 (with key 25) to 39 (with key 0).

Note also that, since in your system the key space is smaller than the plaintext space, it is not perfectly secret even if you use modular arithmetic. As above, a ciphertext of 39 ensures the plaintext is between 14 and 39, not below (there is no key that encrypts 13 to 39).