I figured there had to be a map/grep in there somewhere. Being a "Bear of Very Little Brain", map (and to a lesser extent grep) still throw me.

You'd be thinking something like:

   $s = 'hi, there, world, how, are you, today?';
   print join "\n", keys %{{ map {$_ => undef} (split /,\s+/, $s)}};
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And no, I didn't figure that one out by myself :-(

Although, I rather prefer:

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