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for ($ref=0; $ref<$#items; $ref++) {
    for (@{$items[$ref]}) {
        if (!$contains{$_}) { $contains{$_} = [$ref]; } else { push @{
+$contains{$_}}, $ref; }
    }
}

sub get_pairs {
    my %invalids = ();
...
}

@all_matches = get_pairs(2,3,4)