That's fine if you're actually using elements of @a for the lookup, but that rather presupposes that you know where they are to begin with, doesn't it? How about this case?

my @a = ('a'..'d','a'..'d');
my %a; @a{\(@a)} = 0..$#a;
my $x = 'b';
print "$x has an index of $a{\$x}\n";

b has an index of
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Better for the most general case would be to make a lookup hash keyed by the values of the array, with each value of the hash being a reference to an array to which you push the indices from the original array. That way, when you key the hash with an array value, you get every location in the array where that value occurred.