Short answer: if you are going to need this calculation only once, use a loop. If you need to do this kind of calculation all the times, refactor your code.

Here goes the longer explanation.

A loop is the fastest way to get the index of an array item, if you need it only once. Getting that information from a hash is faster, but filling that hash is not going to be faster than a simple loop.

LOOP:
for (0 .. $#array) {
    if ($array[$_] eq $searched) {
        print "$searched found at $_ \n";
        last LOOP;
    }
}
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Notice that this approach would find the first occurrence of the item, therefore giving you that index. Conversely, using a hash would give you the last occurrence of the same item, unless you take it into account.

For example, this will hold the last index of each item.

my %hash;

$hash{array[$_]} = $_ for 0 .. $#array;
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To get all the indices of each item, you should do this, instead:

push @{ $hash{array[$_]} } ,  $_ for 0 .. $#array;
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Now, for the long array, the one that you can't afford to store into a hash, because it would more than double your memory occupation.

If you are in such a situation, you can't build a hash, and looping through the array is going to take long as well. Therefore, you have to refactor your code, using a different data structure or removing the need for knowing the index of a given item. QED.

HTH