use strict;
use warnings;
use Devel::Peek;
use Encode;
use utf8;

my $utfstring = "123 \x{439}\x{439}\x{439}\x{439}";
my ($ascii_but_utf, undef) = split ' ', $utfstring;

my $bytestring = encode ("UTF-8", "\x{444}\x{444}\x{444}\x{444}");
my $mixedstring = "$ascii_but_utf$bytestring"; # simulate The Unicode Bug here
binmode STDOUT, ":utf8";

Dump $mixedstring;
print "UTF IS ON\n" if utf8::is_utf8($mixedstring);
print "LENGTH DIFFERS\n" if length($mixedstring) != bytes::length($mixedstring);

open my $f, ">", "test2";
binmode $f;
syswrite $f, $mixedstring or die;
print "ALL OK\n";

__END__
SV = PV(0x1d6eb48) at 0x1c7fab8
  REFCNT = 1
  FLAGS = (PADMY,POK,pPOK,UTF8)
  PV = 0x1d79820 "123\303\221\302\204\303\221\302\204\303\221\302\204\303\221\302\204"\0 [UTF8 "123\x{d1}\x{84}\x{d1}\x{84}\x{d1}\x{84}\x{d1}\x{84}"]
  CUR = 19
  LEN = 24
UTF IS ON
LENGTH DIFFERS
ALL OK