#!/usr/bin/perl

use strict;

my @input = qw(a b c d e d e f);

# %seen is a hash used to count the number of times
# an element appears in the array.  @result contains
# the results of removing the duplicates.
my (%seen, @result);

foreach (@input)
{
        $seen{$_}++;
}

foreach (@input)
{
        # Snag the elements which appear only once.
        # Use the original elements of the input array
        # as hash keys rather than keys(%seen) to
        # preserve ordering
        if ($seen{$_} == 1)
        {
                push @result, $_;
        }
}