Whenever tuples of elements in an array have some relationship to each other.... think hashes.

It sounds like the data structure you want is a hash of hashes, like so:

$hostdata = {
    host1 => {
        user1 => "info1",
    },
    host2 => {
        user2  => "info2",
        user17 => "info39",
    },
    # ...
};
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In this case, assuming userM exists on hostN, to get the info for that user you'd just use:

$info = $hostdata->{hostN}->{userM};         # the second "->" is optional

There are issues like this, such as autovivification, but you can read up on that if you like. (You should, actually.)

And as for your original question: assuming data integrity, that is, the array size is a multiple of three, you can just run an index from 0 up, incrementing it by 3 every time, and return a result if ($array[$i] eq $host && $array[$i + 1] eq $user) .