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

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

@data = uniq @data, @rows;

use strict;
use warnings;
...
    listutil => sub {my $f = listutil(\@first, \@second)},
    seengrep => sub {my $f = seengrep(\@first, \@second)},
});

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

my @list_A = (1..1000);
my @list_B = (1..1000);
...
        do_something($element_A, $element_B);
    }
}

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

foreach my $row (@rows) {
    my $found = 0;
...
    }
    push @data, $row if $found;
}

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