my @keywords;
{ #closure so %temphash gets garbage collected
	my %temphash;
	foreach ( map {lc} qw(this that other foo THIS bar)) {
		next if exists $temphash{$_};
		$temphash{$_} = undef;
		push @keywords, $_;
	}
}
print join " ", @keywords;

___OUTPUT____
this that other foo bar