Another way to do it since davido implemented my first idea:

use warnings;
use strict;

my @array1 = qw(
        test
        test1
        test
        test2
        test2
        test
        );
        
my @array2 = qw(
        test1
        test2
        test2
        );
my @result;

@array1 = sort @array1;
@array2 = sort @array2;

while (@array1 && @array2) {
    if ($array1[0] lt $array2[0]) {
        shift @array1;
    } elsif ($array2[0] lt $array1[0]) {
        shift @array2;
    } else {
        push @result, shift @array1;
        shift @array2;
    }
}

print join "\n", @result;
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Prints:

test1
test2
test2
[download]