The sub is being used as a comparitor. See the documentation for sort SUBNAME LIST

#! perl -slw
use strict;
$a=$b='unused';
sub uniq { print "unique($a,$b):",@_; 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;

## uniq is used a comparator here
print sort uniq @array;

__DATA__
unique(unused,unused):ABCDEFGHIJKLMNOPQRSTUVWXYZZYXWVUTSRQPONMLKJIHGFE
+DCBA
ABCDEFGHIJKLMNOPQRSTUVWXYZ
unique(A,B):
unique(C,D):
unique(E,F):

<SNIP>

unique(K,J):
unique(I,H):
unique(G,F):
unique(E,D):
unique(C,B):
ABCDEFGHIJKLMNOPQRSTUVWXYZZYXWVUTSRQPONMLKJIHGFEDCBA
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You need to use the full ampersand and brackets to avoid it.

#! 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
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