Cut vertically through 1 whole outer layer of each leek and pull it off gently. Cut at least 7 inches off the top of each leek. Rinse outer layers and tops.
Trim off and discard leek root ends. Split white parts of leeks in half lengthwise and rinse thoroughly to remove dirt between layers. Tie each leek half in the center with a cotton string.
In a 5- to 6-quart pan over high heat, bring about 3 quarts water to boiling. Add leek outer layers and about 2/3 of the green tops (select the freshest-looking). Cook just until wilted, about 30 seconds; lift out with tongs and immerse in ice water. Discard remaining tops.
Add tied leek halves to water and simmer, uncovered, until tender when pierced, 5 to 7 minutes; lift out with a slotted spoon and immerse in ice water. When cool, drain well.
Add potatoes to boiling water, cover, and simmer just until tender when pierced, 20 to 25 minutes. Drain and cool. Cut into 1/2-inch-thick slices.
Rinse salmon and pat dry. Sprinkle each piece with a crushed juniper berry, salt, and pepper.
Tear leek outer layers lengthwise into strips about 1/4 inch wide. Prepare wrappers and wrap salmon. Cross two leek strip. Unfold leek tops and lay flat, overlapping to make a rectangle. Center a salmon fillet on a leek base slightly wider than the length of the fish. Fold leek greens over fish to enclose. Tie packet with leek strips to secure. Repeat to wrap remaining salmon.
Pull back cornhusks; don't remove. Discard silk. Push husks back in place.
Prepare barbecue for indirect cooking. If using charcoal briquets, mound and ignite 60 briquets on the firegrate of a barbecue (20 to 22 in. wide) with a lid; open vents. When briquets are dotted with gray ash, in 15 to 25 minutes, push equal amounts to opposite sides of the firegrate. Add 5 more briquets to each mound of coals (10 coals).
If using a gas barbecue, it must be equipped with a control to regulate heat in the center of the grill. To heat the barbecue, cover and turn on high for about 10 minutes. Turn off heat in center of barbecue, but leave heat on opposite sides on high.
Lay fish in center of grill, not over direct heat. Place potato slices and corn directly over heat. If space permits, also put leek halves directly over heat (otherwise, lay next to fish). Cover barbecue, open vents, and cook 8 minutes. Turn vegetables over and cook until they are hot and lightly browned, 4 to 6 minutes longer. Cook fish until a thermometer inserted in thickest part reaches 125°, about 16 minutes total. Transfer foods to a platter as they're cooked.
Open packets. Drizzle hazelnut oil over fish, potatoes, and corn. Spoon salmon caviar onto fish. Season to taste with salt and pepper.