Preheat a grill or grill pan until hot, or prepare oven broiler. Bring a medium saucepan of salted water to a boil. Cut off asparagus ends to within 3 1/2 inches from tips; reserve bottoms for another use. Place tips in boiling water; cook until bright green but still crunchy, about 1 minute. Prepare an ice-water bath. Drain asparagus; transfer to an ice-water bath to stop cooking. Drain asparagus again in a colander, and set aside.
Whisk together soy sauce and sugar until dissolved; set aside. Cut scallions into 3 1/2-inch lengths, julienned lengthwise; set aside.
Slice tenderloin into 1/4-inch-thick pieces. Place one slice between two pieces of plastic; pound lightly to an even thickness. Remove plastic; trim into 2-by-5-inch rectangle. Repeat with remaining beef.
Dip a piece of beef in soy-sauce mixture, and place on a clean surface. Season with pepper. Place 1 piece of scallion and 2 asparagus tips across 1 end of beef, so vegetables extend over edges; roll. Set aside. Repeat with remaining beef and vegetables.
Grill or broil negamaki, brushing with sauce and turning, until slightly charred and medium rare, about 2 minutes. Serve.