I've been looking for an explicit discussion of this.

Me too... I also found a few similar mentions in perlre:

Capture group contents are dynamically scoped and available to you outside the pattern until the end of the enclosing block or until the next successful match, whichever comes first. ... These special variables, like the %+ hash and the numbered match variables ($1, $2, $3, etc.) are dynamically scoped until the end of the enclosing block or until the next successful match, whichever comes first.

But I think the language regarding // is pretty direct, if IMHO misleading: "the last successfully matched regular expression is used". No mention of scope anywhere even near that statement...