Actually the variables are SELECT_LINE and _SELECT_KEY. What is confusing is that for the first time an environment variable (_SELECT_KEY) has been defined with a leading underscore. In all previous version the leading underscore was reserved for internal variables. Of course, there is little practical difference - only the IF DEFINED test behaves differently. One could always define one's own environment variable (ev) with the same name as an internal variable (iv), and once done, only the value of the ev was accessible (until UNSET was used to undefine the ev, making the value of the iv accessible again).
The debugger allows viewing / monitoring iv-s, not just ev-s.
Rex: @execarray reports a count in _execarray which is an iv - why is that one easier for the parser than using iv-s for @SELECT and @LINES reports?