$#{@1}
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would die in the same way. It does not.

No, it wouldn't, as it doesn't use a symbolic reference...

If you take the time to dump or peek at @1, you'll see that it exists as empty array...as do all @NUM I tested.

My guess was, that @1 exists as a side effect of $1 beeing a default-variable and the "multi-type" nature of perl variables. I'd love to be corrected/educated on this rather blunt assumption I wasn't able to backup with my first dive into Devel::Peek and the like.