#! perl -slw
use strict;
sub uniq { my %x; @x{@_} = (); keys %x }

my @array = ( 'A' .. 'Z', reverse 'A' .. 'Z' );

## Explicit sort block works as expected
print sort { $a cmp $b } uniq @array;

## must use &sub(@args); here or uniq is used as a comparator.
print sort &uniq(@array);

__DATA__
ABCDEFGHIJKLMNOPQRSTUVWXYZ
ABCDEFGHIJKLMNOPQRSTUVWXYZ