Use Unicode. Perl is quite good at that

$ perl -MDP -we'my@x=("",undef,"1","123","2","\xff","\x{00ff}");DPeek 
+for map{$_->[1]}sort{$a->[0]cmp$b->[0]}map{[$_//"\x{1ffff}",$_]}@x'
PV(""\0)
PV("1"\0)
PV("123"\0)
PV("2"\0)
PV("\377"\0)
PV("\377"\0)
UNDEF
[download]

And chr(255) is *not* per definition an y with two dots. That is only the case in (encodings supported by perl (cp1252, cp1254, cp1258, hp-roman8, iso-8859-1, iso-8859-9, iso-8859-14, iso-8859-15, iso-8859-16, and UTF-7. If you don't specify the encoding or (lord forbids) *assume* any of the just listed, chr(255):

  7bit-jis                       \xFF
  cp1006                         ﹽ      ARABIC SHADDA MEDIAL FORM
  cp1026                                APPLICATION PROGRAM COMMAND
  cp1047                                APPLICATION PROGRAM COMMAND
  cp1250                         ˙      DOT ABOVE
  cp1251                         я      CYRILLIC SMALL LETTER YA
  cp1252                         ÿ      LATIN SMALL LETTER Y WITH DIAERESIS
  cp1254                         ÿ      LATIN SMALL LETTER Y WITH DIAERESIS
  cp1256                         ے      ARABIC LETTER YEH BARREE
  cp1257                         ˙      DOT ABOVE
  cp1258                         ÿ      LATIN SMALL LETTER Y WITH DIAERESIS
  cp37                                  APPLICATION PROGRAM COMMAND
  cp424                                 APPLICATION PROGRAM COMMAND
  cp437                                 NO-BREAK SPACE
  cp500                                 APPLICATION PROGRAM COMMAND
  cp737                                 NO-BREAK SPACE
  cp775                                 NO-BREAK SPACE
  cp850                                 NO-BREAK SPACE
  cp852                                 NO-BREAK SPACE
  cp855                                 NO-BREAK SPACE
  cp856                                 NO-BREAK SPACE
  cp857                                 NO-BREAK SPACE
  cp858                                 NO-BREAK SPACE
  cp860                                 NO-BREAK SPACE
  cp861                                 NO-BREAK SPACE
  cp862                                 NO-BREAK SPACE
  cp863                                 NO-BREAK SPACE
  cp865                                 NO-BREAK SPACE
  cp866                                 NO-BREAK SPACE
  cp869                                 NO-BREAK SPACE
  cp875                                 APPLICATION PROGRAM COMMAND
  cp932                          
  cp936                          
  cp949                          
  cp950                          
  gsm0338                        ?      QUESTION MARK
  hp-roman8                      ÿ      LATIN SMALL LETTER Y WITH DIAERESIS
  iso-2022-jp                    \xFF
  iso-2022-jp-1                  \xFF
  iso-2022-kr                    \xFF
  iso-8859-1                     ÿ      LATIN SMALL LETTER Y WITH DIAERESIS
  iso-8859-10                    ĸ      LATIN SMALL LETTER KRA
  iso-8859-13                    ’      RIGHT SINGLE QUOTATION MARK
  iso-8859-14                    ÿ      LATIN SMALL LETTER Y WITH DIAERESIS
  iso-8859-15                    ÿ      LATIN SMALL LETTER Y WITH DIAERESIS
  iso-8859-16                    ÿ      LATIN SMALL LETTER Y WITH DIAERESIS
  iso-8859-2                     ˙      DOT ABOVE
  iso-8859-3                     ˙      DOT ABOVE
  iso-8859-4                     ˙      DOT ABOVE
  iso-8859-5                     џ      CYRILLIC SMALL LETTER DZHE
  iso-8859-9                     ÿ      LATIN SMALL LETTER Y WITH DIAERESIS
  koi8-f                         Ъ      CYRILLIC CAPITAL LETTER HARD SIGN
  koi8-r                         Ъ      CYRILLIC CAPITAL LETTER HARD SIGN
  koi8-u                         Ъ      CYRILLIC CAPITAL LETTER HARD SIGN
  MacArabic                      ے      ARABIC LETTER YEH BARREE
  MacCentralEurRoman             ˇ      CARON
  MacChineseSimp                 …      HORIZONTAL ELLIPSIS
  MacChineseTrad                 …      HORIZONTAL ELLIPSIS
  MacCroatian                    ˇ      CARON
  MacCyrillic                    €      EURO SIGN
  MacFarsi                       ے      ARABIC LETTER YEH BARREE
  MacGreek                              SOFT HYPHEN
  MacHebrew                      |      VERTICAL LINE
  MacIcelandic                   ˇ      CARON
  MacJapanese                    …
  MacKorean                      …
  MacRoman                       ˇ      CARON
  MacRomanian                    ˇ      CARON
  MacRumanian                    ˇ      CARON
  MacSami                        ǩ      LATIN SMALL LETTER K WITH CARON
  MacTurkish                     ˇ      CARON
  posix-bc                       ~      TILDE
  UTF-7                          ÿ      LATIN SMALL LETTER Y WITH DIAERESIS
  viscii                         Ữ      LATIN CAPITAL LETTER U WITH HORN AND TILDE

Touché.

$ perl -MDP -we'my@x=("",undef,"1","123","2","\xff","\x{00ff}");DPeek for map{$_->[1]}sort{$a->[0]cmp$b->[0]}map{[$_//"\x{1ffff}",$_]}@x'

Not sure what you're trying to prove with this. Do you think U+1ffff is the biggest Unicode character?

And chr(255) is *not* per definition an y with two dots.

Which is exactly why I said I was *hoping* it would get replaced with something less than 255. People around here need less sensitive nerd-rage triggers.

It would seem that deaccent()would modify the data to a sub-255 value, leaving a single 255 in the Schwartian Transform as a viable sort max key -- as noted above, this should be proven before deployed.

As to your other note, Unicode characters "above 255" are actually multi-byte sequences whose individual bytes still cannot exceed the architectural limitation of chr(255) so I question that perceived vulnerability.

Perl strings are sequences of Unicode code points, not sequences of bytes. (Well, I think they're stored internally as plain bytes if possible.)

use Encode qw( encode_utf8 );
my $x = chr(1 << 63);
print length($x), "\n";
print length(encode_utf8($x)), "\n";
print "yep\n" if $x gt chr(255);
[download]

Output:

Use of code point 0x8000000000000000 is deprecated; the permissible ma
+x is 0x7FFFFFFFFFFFFFFF at foo line 2.
1
13
yep
[download]

Going back to my attitude when I was a C programmer: It pays to know how your compiler thinks. (Needs adjustment for application to modern use of Perl, but the sentiment is the same.)

Adding/replacing these three lines into my original script above:

use Encode qw( encode_utf8 );
my $x = chr(1 << 63);
my @Unsorted = ( 'Dog', 'Cat', 'Bird', undef, $x, 'Elephant', undef, '
+Lizard' );
[download]

Yields:

S:\Steve\Dev\PerlMonks\P-2017-06-12@0734-sort-undef>perl .\sort011.pl
----------------------------------------------------------------------
+---------
Original:
----------------------------------------------------------------------
+---------
Dog
Cat
Bird
(undef)
Wide character in print at .\sort011.pl line 58.
 ÇêÇÇÇÇÇÇÇÇÇÇ
Elephant
(undef)
Lizard
----------------------------------------------------------------------
+---------
----------------------------------------------------------------------
+---------
Custom Sort:
----------------------------------------------------------------------
+---------
Bird
Cat
Dog
Elephant
Lizard
(undef)
(undef)
Wide character in print at .\sort011.pl line 58.
 ÇêÇÇÇÇÇÇÇÇÇÇ
----------------------------------------------------------------------
+---------

S:\Steve\Dev\PerlMonks\P-2017-06-12@0734-sort-undef>
[download]

A string of chr(255)bytes longer than the longest item in the original array still fails to sort to the bottom; knowing that Unicode characters are stored differently than old-fashioned ASCII strings empowers the Perl programmer to make a better choice.

Thank you for the information!

I'd upvote the post, but there isn't any point, as it's Anonymous Monk.