Have a look into the results of

#!/usr/bin/perl

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

binmode STDOUT, ":utf8";

my $string1 = "123\x{444}\x{444}\x{444}\x{444}";
_display ($string1, 'STRING1' );

my $utfstring = "123 \x{439}\x{439}\x{439}\x{439}";
_display ($utfstring, 'UTF_STRING' );

my ($ascii_but_utf, undef) = split ' ', $utfstring;
_display ($ascii_but_utf, 'ASCII_BUT_UTF' );

#my $bytestring = encode ("UTF-8", "\x{444}\x{444}\x{444}\x{444}");
my $bytestring = "\x{444}\x{444}\x{444}\x{444}";
_display ($bytestring, 'BYTESTRING' );

my $mixedstring = "$ascii_but_utf$bytestring"; # simulate The Unicode 
+Bug here
_display ($mixedstring, 'MIXEDSTRING' );

print "MIXEDSTRING and STRING1 are supposed to be identical...\n";

exit;

###############

sub _display {
   my ($string, $name) = @_;

   print "$name:\n";
   Dump $string;

   my $l1 = length($string);
   my $l2 = bytes::length($string);
   if ($l1 != $l2) {
       print "LENGTHs DIFFERS: length: $l1, bytes: $l2\n" 
   }

   print "UTF IS ON\n" if utf8::is_utf8($string);
   print "\n";
}
[download]

and then check the difference you see for BYTESTRING when running

my $bytestring = encode ("UTF-8", "\x{444}\x{444}\x{444}\x{444}");

versus

my $bytestring = "\x{444}\x{444}\x{444}\x{444}";

The Encode documentation has an Caveat about it:

CAVEAT: When you run "$octets = encode("utf8", $string)", then $octets might not be equal to $string. Though
both contain the same data, the UTF8 flag for $octets is always off. When you encode anything, the UTF8 flag
on the result is always off, even when it contains a completely valid utf8 string. See "The UTF8 flag" below.

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