My quick reaction is that the default should be /a...++
If there's a way to "use re 'legacy_unicode'"...
!? (/me wondering about --)
There's enough perl -e one-line filters in shell scripts to make just require an additional 'use' statement a daunting and painful task, merely to retain the originally intended behaviour. So I'd say stick to the ASCII semantics by default, but allow modifiers and use-clauses (file-only scope!?) to modify the behaviour.
With Unicode proper it seems to be useful to add a single new \w escape that includes latin1 umlauts. Similar arguments can be made for extended whitespace escapes.
But basically it's impossible to add enough escapes for all kinds of sensible subsets of digit-class characters, whitespace, word-characters or alphanumerical characters: The only sane way to cope is switching to explicit character classes early in our regex.
And we probably shouldn't even consider wasting an unused character for a new alternative \w-style escape adding latin1-umlauts (says a German, who is using those umlauts all the time).
adding another 0.01€ and staying firmly in the ASCII semantics camp,