poc1.pl:

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

my $string = "123\x{444}\x{444}\x{444}\x{444}";
binmode STDOUT, ":utf8";

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

open my $f, ">", "test1";
binmode $f;
syswrite $f, $string or die;
print "ALL OK\n";
__END__
SV = PV(0x258cb78) at 0x25b7bb0
  REFCNT = 1
  FLAGS = (PADMY,POK,pPOK,UTF8)
  PV = 0x25aad60 "123\321\204\321\204\321\204\321\204"\0 [UTF8 "123\x{
+444}\x{444}\x{444}\x{444}"]
  CUR = 11
  LEN = 16
UTF IS ON
LENGTH DIFFERS
Wide character in syswrite at poc1.pl line 16.
[download]

poc2.pl:

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($mixedstring) != bytes::length($mix
+edstring);

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{d
+1}\x{84}"]
  CUR = 19
  LEN = 24
UTF IS ON
LENGTH DIFFERS
ALL OK
[download]

After __END__ of each file I appended program output.

Let's ignore for the moment the fact that those strings completely different and contains different characters and the fact that at some point of time one of the strings was interpreted as latin-1 etc

So, in both cases strings have UTF-8 bit set. They both have non ASCII-7bit octets. Their length() and bytes::length differs. And I expect those strings should behave same way

Question is why in one case string was treated as 'wide character string' and syswrite terminated the program. In other case all was working fine

p.s reproduced on perl 5.10 and perl 5.14 (linux)

UPD: escaped utf chars in sourcecode, as perlmonks eats it

UPD: SOLVED: http://www.perlmonks.org/?node_id=1032996 http://www.perlmonks.org/?node_id=1033006