my %seen;
my @union = grep {! $seen{$_}++} @rows, @data;

##</code><code>##

my %seen;
@seen{@data, @rows} = ();
@data = keys %seen;

##</code><code>##

@data = uniq @data, @rows;

##</code><code>##

use strict;
use warnings;
use Test::More;
use Benchmark qw(cmpthese);
use List::Util qw(uniq);

my @first = (1 .. 1000);
my @second = (300 .. 700);

sub slice {
    my %seen;
    @seen{@{$_[0]}, @{$_[1]}} = ();
    return [keys %seen];
}

sub listutil {
    return [uniq(@{$_[0]}, @{$_[1]})];
}

sub seengrep {
    my %seen;
    return [grep {! $seen{$_}++} @{$_[0]}, @{$_[1]}];
}

is_deeply [sort {$a <=> $b} @{slice(   \@first, \@second)}],
          [sort {$a <=> $b} @{listutil(\@first, \@second)}],
          "They matched.";

is_deeply [sort {$a <=> $b} @{listutil(\@first, \@second)}],
          [sort {$a <=> $b} @{seengrep(\@first, \@second)}],
          "They matched again.";

done_testing();

cmpthese (-5, {
    slice    => sub {my $f = slice   (\@first, \@second)},
    listutil => sub {my $f = listutil(\@first, \@second)},
    seengrep => sub {my $f = seengrep(\@first, \@second)},
});

##</code><code>##

ok 1 - They matched.
ok 2 - They matched again.
1..2
           Rate seengrep    slice listutil
seengrep 3555/s       --     -16%     -17%
slice    4233/s      19%       --      -1%
listutil 4297/s      21%       2%       --

##</code><code>##

my @list_A = (1..1000);
my @list_B = (1..1000);


# Exhibit A:

foreach my $element (@list_A, @list_B) {
    do_something($element);
}


# Exhibit B:

foreach my $element_A (@list_A) {
    foreach my $element_B(@list_B) {
        do_something($element_A, $element_B);
    }
}

##</code><code>##

foreach my $row (@rows) {
    push @data, $row unless $row ~~ @data;
}

##</code><code>##

foreach my $row (@rows) {
    my $found = 0;
    foreach my $element (@data) {
        if ($row =~ m/\Q$element/) {
            $found = 1;
            last;
        }
    }
    push @data, $row if $found;
}

##</code><code>##

foreach my $element(@data, @rows) {
    $seen{$_} = ();
}