This recipe has been updated and may differ from what was originally published or broadcast.
Place the shrimp in a large (8-inch-diameter × 5-inch-high) saucepan and add 8 cups of cold water, the Pernod, and 1 tablespoon salt. Turn the heat to medium high and cook the shrimp uncovered, stirring occasionally, for 8 to 10 minutes, until just firm and pink. Drain in a colander.
Place half the shrimp in a large bowl. Cut the rest of the shrimp sideways through the back with a sharp paring knife and add them to the bowl as well. Allow to cool for 15 minutes. Stir in the lemon juice, 2 teaspoons salt, and 1 teaspoon pepper. Add the celery, scallions, tarragon, and mayonnaise, and combine. Cover and refrigerate for at least 2 hours for the flavors to meld.
Transfer the salad to a serving bowl, sprinkle with extra tarragon and salt, and taste for seasonings. Serve cold or at room temperature.