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   $s = 'hi, there, world, how, are you, today?';
   print join "\n", keys %{{ map {$_ => undef} (split /,\s+/, $s)}};

   $s = 'hi, there, world, how, are you, today?';
   print (join "\n", (grep ! $t{$_}++, (split /,\s+/, $s)));