Cook pork: Put pork, water, salt, and garlic in a 5- to 6-quart wide heavy pot and bring to a boil, skimming froth. Reduce heat and simmer, uncovered, stirring occasionally, until all of water is evaporated, 1-1/2 to 2 hours, then continue to cook, allowing pork to fry in its own fat, turning occasionally, until browned, 5 to 10 minutes. Remove from heat.
Make sauce while pork simmers: Cover whole tomatillos with water in a 2- to 3-quart saucepan and simmer, uncovered, turning over occasionally, until tender, 10 to 20 minutes.
While tomatillos simmer, heat comal over moderate heat until hot, 2 to 3 minutes.
Toast chiles, 2 or 3 at a time, pressing each with tongs and turning once, until color changes slightly (some will inflate), about 10 seconds per batch. Transfer as toasted to a bowl.
Open chiles (wear protective gloves), discarding stems and seed cluster, then tear chiles into large pieces and put into a large bowl.
Transfer tomatillos to a blender with a slotted spoon, then add 2 cups tomatillo-cooking water (if blender is too full, blend in batches), reserving remainder. Add chile pieces and salt and blend until very smooth, about 2 minutes (use caution when blending hot liquids).
Heat sauce with pork: Add puree to browned pork in pot and bring to a simmer over moderate heat, stirring and scraping up brown bits from bottom of pot. Add oregano and simmer, uncovered, stirring occasionally and adding more tomatillo-cooking liquid or water if sauce becomes too thick, 10 minutes.
Cooks' note: Pork (with sauce) can be made 1 day ahead and cooled completely, uncovered, then chilled, covered. Reheat, covered, over moderately low heat, stirring occasionally and adding more liquid if necessary.