It's not just Perl -- it's also CPAN modules, particularly XS modules. And I agree -- it's foolhardy to pretend that the SVf_UTF8 flag doesn't exist. It's almost impossible to troubleshoot UTF-8 problems in a large system without snooping it. The system is prone to silent failure, and when something goes wrong and you need to track down where the silent failure originates, you need to look at that flag.

(Die $YAML::Syck::ImplicitUnicode, die die die.)