Here's a quick hash-as-unique-array technique:

my %hash;
for (qw(first second first third second fourth)) {
    exists $hash{$_} or $hash{$_} = keys %hash;  # Put stuff in like t
+his
}
print "$_\n" for sort {$hash{$a} <=> $hash{$b}} keys %hash; # Get stuf
+f out like this
[download]