#!/usr/bin/perl

use strict;
use warnings;

my @list = qw (a a b b c c d d);

print "\nAssigning list:\n";

for my $i (0 .. 1) {
    my @unique    = ($i)? removeDuplicates (@list): &removeDuplicates (@list);
    my $ampersand = ($i)? "": "&";
    print $ampersand, $_, "\n" for (sort @unique);
    print "=\n";
}

print "\nUsing in for loop:\n";

for my $i (0 .. 1) {
    if ($i) {
        for my $character (sort removeDuplicates (@list)) {
            print $character, "\n";
        }

    } else {
        for my $character (sort &removeDuplicates (@list)) {
            print "&", $character, "\n";
        }
    }
    print "=\n";
}

sub removeDuplicates {
    my %hash;
    @hash{@_} = 1;
    return keys %hash;
}

##</code><code>##

user@host:~/scripts/test$ ./unique.pl 

Assigning list:
&a
&b
&c
&d
=
a
b
c
d
=

Using in for loop:
&a
&b
&c
&d
=
a
a
b
b
c
c
d
d
=
user@host:~/scripts/test$