I like the ideas of having /a, /u and /l modifiers. And while I have nothing against the fourth one, I question its value.

The 5.8 behavior is rather confusing, and I remember a lengthy discussion with ikegami where we tried to figure out the details, and it took us quite some time. Also it seemed to have changed between different maintenance releases of 5.8 (not quite sure on this one), so I think that nearly nobody relies willingly on these semantics.

I'd also love to see a way to set the default in a pragma. I envision a pragma that enables as much Unicode behavior as possible, something like use utf8::all; or so, which imports utf8.pm, use open ':encoding(utf8)'; and the pragma that enables /u by default.