Preheat the oven's broiler and set the oven rack about 6 inches from the heat source. Line a baking sheet with aluminum foil.
Using a mortar and pestle or spice grinder, grind the peppercorns, chopped garlic, lime zest, and salt until the peppercorns are finely ground. Rub salmon fillets with olive oil, then place onto the prepared pan, skin side down. Rub spice mixture onto top of salmon fillets.
Broil until the salmon is flaky and no longer pink in the center, 10 to 15 minutes. Drizzle with soy sauce and serve.