Keeping order (but only keeping the last seen element of a string):

#!/usr/bin/perl -w

use strict; 

my ($i, $keywords, @finalwords);
%$keywords = map { lc($_) => $i++} qw/foo bar bat hello foo HELLO/;

print join " ", @finalwords = sort {$keywords->{$a} <=> $keywords->{$b
+}} keys %$keywords;
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My comment here is that you can use the hash's value to preserve order as well. Comments welcome.

    --jaybonci