One would hope you didn't get that code here since it's doing a linear scan of @group1 for every element of @group2. The way that's in the FAQ is in the FAQ for a reason.

Update: Aaah, you want the union to have only distinct copies of the elements.

my @distinct_union, %seen;

for my $e ( @group1, @group2 ) {
  push @distinct_union, $e unless $seen{ $e }++;
}
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