I'm not using an utf8 locale, nor does I use 'use utf8'.

The comment about 'use utf8' here makes me think it's just possible you are mistaken. "Using a utf8 locale" isn't a perl thing, it's an operating system thing, and several OS's have taken to setting utf8 locales as a default. Check your environment variables LANG, LC_CALL, LC_CTYPE, and LANGUAGE.

If that's not it, you'll have to dig a little bit into what happens to this variable; at what point does it get the utf8 flag turned on?