There's never been a setting like that that I can recall. Maybe you're confusing it with setting $* (now deprecated). Or maybe some other language that (falsely¹) claims to have Perl-compatible regular expressions.

-- Randal L. Schwartz, Perl hacker

¹ Unless there's an embedded Perl interpreter, it can't be completely compatible, because of the use of variable interpolation and embedded Perl code in a regex.