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
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--

flounder