You're correct about the documentation, at least for Perl v5.16 and up (in which the entire given/when/smartmatch feature is "experimental"), and Switch statements could mention the behaviour of last within a given for clarity.
But last without a label always breaks out of the innermost loop, so I still don't find that behaviour surprising and your observation that "the 'last' statement in the switch statements controlled the flow within the switch statement" is not quite right, since in your examples you're not inside a "switch" (given), you're inside two for loops.
The only argument one might make is that that behaviour is inconsistent and last should also break out of a given (but according to your original post you don't expect that behaviour).
I don't understand what you mean about a "masked loop statement". Switch is deprecated and has nothing to do with Switch statements (given/when) except the shared name of the feature.