In that case, the question is "why does the scalar have the utf8 flag set". If it came from a filehandle, that question is equvilent to "why is the filehandle set to be in a utf8 encoding". A binmode will probably solve your problems here.

If it didn't come from a filehandle, or marking the FH as having binary data is not a good thing, you can use $wasutf8 = Encode::_utf8_off($string);

In case you wondered, my general rule is that giving the runtime more information about what's going (by making sure the utf8 bit is set correctly on scalars, or the encoding is set correctly on filehandles) is better then forcing it to do what you want, when it has other ideas (by using bytes).

(That's not a hard and fast rule, of course...)