Yes, you are correct. How embaracing. The timing remains about the same though.

Union_1 96235
Union_2 96235
            (warning: too few iterations for a reliable count)
         s/iter   sorted unsorted
sorted     4.39       --     -66%
unsorted   1.48     196%       --
Union_1 96235
Union_2 96235
Union a b c d g e f
Union a b c d e f g
           Rate unsorted   sorted
unsorted  202/s       --     -92%
sorted   2511/s    1144%       --
Union a b c d g e f
Union a b c d e f g

sub union_1
{
  my $count = shift;
  $count--;
  my %saw;

  grep(0 == $saw{$_}++, @_);
}

sub union_2
{
  my $self = [@_];
  my ($lowest_arr, $lowest, $next);
  my @ret = ();
  my $elements = scalar @_;

  while (1) {
    my $count = 0;
    my $work = 0;
    my $lowest;
    foreach my $a (@$self)
    {
      next unless (@$a);
      if (!defined($lowest) or ($a->[0] le $lowest))
      {
        $lowest = $a->[0];
      }
    }
    foreach my $a (@$self)
    {
      next unless (@$a);
      if ($a->[0] eq $lowest)
      {
        shift @$a;
        $work++;
      }
    }
    return @ret unless $lowest;
    push(@ret, $lowest);
  }
}
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