UTF-8 characters getting converted in plugin

I'm updating a plugin I wrote (PagedArchives) to run with MT5. All went well and I got it working, however if I include UTF8 characters in a post, they get converted somehow to ASCII.

I'm providing a BuildFile handler, which uses RawContent from the arguments passed. As far as I can tell, the RawContent contains the UTF8 characters correctly, however when I do a pattern match on it and save the matched string, the UTF8 bytes in this matched string have been converted to ASCII. From what I have read on the subject, Perl should be doing no conversion at all, and simply treating the string as a sequence of bytes.

I have successfully gotten it to output the UTF8 characters properly by calling utf8::encode($string); on the strings before writing them out, however this is just a hack.

I've also tried, without success: declaring "use utf8;" at the top, and calling "setlocale" to set the current locale to use utf8.

So, I'm not sure what the proper way to do this is.