Using keys in this way causes stringification of values. If your arrays have objects or references in them, this will bite. Also note that that 2 for 1 map was slow until Perl 5.6.1. The following is therefore how I do that:

sub unique_merge {
  my %seen;
  grep !$seen{$_}++, @_;
}
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Side benefit? I don't lose the order of the incoming elements. :-) But, as with all hash approaches, my definition of "unique" is "stringifies the same". That is not always the appropriate definition to use.