...everything left of the \K is not included in $& and so cannot be zapped by substitution: it is "kept safe" from substitution.

Well said...

Given this, I might read perlre as "Keep the stuff left of the \K, i.e., don't include it in $&". Put another way, if the stuff left of \K were included in $&, it wouldn't be kept--and I believe this is the essence of your quote above.

Got a great laugh from your Update! Thanks for that...