in reply to JSON, UTF-8 and Filehandles
is_utf8 is used to check if perl's internal representation of a string has been "upgraded" to utf8. That is, the string cannot possibly be represented in ascii, such as your: my $test = "\x{100}\x{2764}";. When you decode a utf8 string it is converted to perl's internal representation. This just happens to be utf8, or something close, but it doesn't have to be and worrying about it is just a waste of time.
Really, you only have to worry about decoding input and encoding output (edit: I had encoding/decoding backwards!). You received mangled text on your files/screen because you are having JSON encode it's results as a utf8-encoded string. JSON returns a string of bytes that are utf8 encoded.
Next, when you write to the filehandle, having set :utf8 on the filehandle, your bytes are automatically encoded to utf8 again. You have doubly-encoded the string and it is quite predictably garbage.
Quoth the utf8 manpage:
Do not use this pragma for anything else than telling Perl that your script is written in UTF-8.
Read the links under the SEE ALSO section of the utf8 docs for more about unicode. I also had this bookmarked, I remember it being pretty good: http://www.ahinea.com/en/tech/perl-unicode-struggle.html