why does sort with uniq not work together

[david2008]

Hi, This is a crosslist from stack overflow The problem is the following

use strict;
use List::MoreUtils qw/uniq/;
use Data::Dumper;

my @x = (3,2);
my @y = (4,3);

print "unique results \n";
print Dumper([uniq(@x,@y)]);

print "sorted unique results\n";
print Dumper([sort uniq(@x,@y)]);

gives as output

unique results 
$VAR1 = [
          3,
          2,
          4
        ];
sorted unique results
$VAR1 = [
          2,
          3,
          3,
          4
        ];

Unfortunately the uniq does not work anymore.
There is an answer from amon that perl uses the sort function form where the first parameter is a function and the second a list, but i still don't understand why it does not execute uniq beforehand and then uses the sort LIST,instead of preferring the sort SUBNAME LIST form.

Amon also suggested to use
sort +uniq @letters;
What does the + mean?
Thanks,
David