I have more thinking to do with your answer, but main (for me) questions i can review.

Pretending it would currently be possible to implement "use utf8_everywhere", it would break every single piece of code that assumes characters are bytes.

Do you mean, that would break any code, which i incorporate to my project if i declare "use utf8_everywhere"? Then, why it does not break now when i use same things (like "use utf8", "use open", binmode...) separately?

Or do you mean, it will break "characters are bytes"-code, if they use "use uf8_everywhere"? Then i agree, but why they need such declaration?

Assume you read four bytes 0x42 0xC3 0x84 0x48, e.g. from STDIN, a file you opened, an inherited file handle, a command line argument or an environment variable. How many characters do these bytes represent? Explain why

If i am declared fully utf8 environment, i treat them being 3 utf8 characters. If i need some other behaviour, i ask it explicitly to handle the source which from those bytes are coming. Where is the problem?