At once and Hereupon

At once adverb - Without delay or hesitation; with no time intervening.

Hereupon and at once are semantically related. In some cases you can use "Hereupon" instead an adverb phrase "At once".

Hereupon adverb - Immediately after this.

At once and hereupon are semantically related. Sometimes you can use "At once" instead an adverb "Hereupon".