$1 and friends are neither global nor lexical variables. They're Weird Magical Things, and they're tied to the optree, not to any scratchpad or symbol table.

The optree is the parsed and processed form of your program that the perl interpreter actually executes. When you have a regex, perl reserves space for the potential match variables and hangs a pointer to them off the optree (sort of a mini-scratchpad. Sort of) which the rest of the code in the lexical scope (and any subsequent inner scopes, at least some times) will access. (The compiler handles the visibility to inner lexical scopes thing--there's still no scratchpads involved)

Because it's just some odd, regex-engine-private memory, it doesn't really behave the same way that other variables do. They don't get explicitly reset--they're just set when a match happens. So if you skip a match, they retain their old values.

This semi-sorta-global behavior also makes for fun with threads--you must mutex-protect any regex with capturing parens in a threaded program when using 5.005-style threads, or when using ithreads with a 5.6.x perl. (5.8.0 unshares them so its safe) This includes ActivePerl's fork-emulation, though since that doesn't actually expose mutexes to do so it's kind of tricky. (Recent ActivePerl releases might have fixed this--check the release notes)