Can I copy all the keys found in the funny tied %+

Yes: my %whatever = %+;

would that be slow?

Slow is a relative term. Did you try? Did you benchmark it? Do you really think the speed of copying %+ matters when compared to running the regexp engine?

I read a mention in passing that %+ and %- are both tied views to the same underlying object. So, can I just grab that object and swap it with a blank one, so now it's mine to keep and a subsequent regex won't clobber it?

Have you looked at Tie::Hash::NamedCapture?