Strain the canned tuna through a fine sieve, pressing with a fork to remove excess liquid. Discard the liquid, then combine the strained tuna, green onions, Japanese mayo, and sriracha sauce in a medium bowl. Mix with a fork until well combined, then set aside.
In a large bowl, season the hot cooked rice with sesame oil, sesame seeds, and salt. Mix well, then cover with a damp towel or lid to keep warm at room temperature.
Place a sheet of gim or nori vertically on a cutting board or bamboo sushi mat, with the rough side facing up and the shiny side facing down. Place 1 cup (or half of the seasoned rice) in the center. Using clean hands or a rice paddle, spread the rice evenly, leaving a 1-inch border along the far edge.
Place two perilla leaves in the center of the rice, overlapping them so they fit within the seaweed sheet. Scoop half of the spicy tuna mixture on top and shape it into a thick log rather than spreading it thinly. Fold the perilla leaves over the tuna to help contain the filling.
Start rolling from the edge closest to you. Firmly roll and tuck the ingredients together without squeezing out the tuna filling. Finish the roll on the exposed 1-inch border, allowing the moisture from the rice to help seal the seam.
Drizzle ½ teaspoon sesame oil over the kimbap roll and spread it evenly over the seaweed with clean hands or a brush. Lightly coat the sides of a sharp knife with sesame oil. Hold the roll firmly with your dominant hand without pressing down, slice into 1-inch pieces, and garnish with sesame seeds.