As an old Perl programmer, I knew immediately what the /o would do. ;-)

I've always loved the recent[1] summary from `perlre`:

>o - pretend to optimize your code, but actually introduce bugs

1: I still think of it as a relatively new change, but it's from 2013: <https://github.com/Perl/perl5/commit/7cf040c1f649790a4040aec...>