Do you know why it was written the way it was

I don't. What I don't understand is why it uses numerical comparison and map instead of the plain and simple

sub unique {
  my %h;
  grep !$h{$_}++, @_;
}
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which, at least on Perl 5.8.8, is faster. (Notably faster if there are many identical elements.)

Any enlightenment would be appreciated.

lodin