What you call "defer" is not a special option for signal handling,

It's not my term, but rather that of the perl documentation: Deferred signals.

This happens completely in user mode, inside the perl executable.

Yes. But it still happens. Thus, Perl defers taking any action on the signal, until the current opcode has completed. That's how "safe signals" work.

But, this is way outside my knowledge base. -- Yes, it seems so.

It always amazes me how little the vast majority of linux users know about the way their OS works.