#!/usr/bin/perl -w
use strict;

my @inputs = qw(first second third fourth fifth first);
my %found;
my @values;

for my $element (@inputs) {
    if (!$found{$element}) {
        push @values, $element;
        ++$found{$element};
    }
}

print "@values\n";

##</code><code>##

first second third fourth fifth