it's a good solution, but I hate to see 'map' used instead of a 'for' unnecessarily, why make perl go to the trouble of building a return array if you're just going to ignore it? .

my @twice = do {
    my %tracker;
    foreach my $a_ref (\@a1, \@a2, \@a3) {
        foreach my $a_value (@$a_ref) { 
            $tracker{$a_value}{$a_ref} = undef;
        }
    }
    grep {2 == keys %{$tracker{$_}}} keys %tracker;
};
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