Because

We already have a separate "continue where it left off last time" feature for regular expressions: //g in a scalar context and pos().

and that is what we'd use. //z would not imply "start where you left off last time". There is no reason for it to because we already have //g and pos(). Why do you think I've been using //g and messing with pos() in my example?

//z would only tell the regex engine to fail [and set pos()] if it looks at the end of the string/buffer. It wouldn't tell it to start where it left off and it wouldn't keep track of where it left off other than by setting pos().

                - tye