"once entered" is correct and usable in written English.
It is used to emphasize that something happened in the past and is no longer happening. For example, "Once entered, the building was silent."
This man once entered prizefights.
Once entered into the food chain, this exposure harms humans.
And errors, once entered, are difficult to detect.
(B) read as follows: "the operation of any trade agreement once entered into; and".
Sally Law: 1) I once entered a contestant named Speedy in a caterpillar race.
I once entered a contestant named Speedy in a caterpillar race.
She once entered a Dolly look-a-like contest and lost.
A ledger inside the case was opened to a page where an accountant once entered players' salaries.
Your diarist, with his infant son, once entered it unwittingly in search of coffee and a loo.
He once entered a snowshoe race up a mountain in Aspen, although he had never before been snowshoeing.
"I used to love to fish," Mr. Broggi said, recalling freshwater tournaments he once entered with his son Hunter.
