How to get the index of an element in an array?

use List::Util qw(first);
$idx = first { $array[$_] eq 'whatever' } 0..$#array;

(List::Util is core)

or

use List::MoreUtils qw(firstidx);
$idx = firstidx { $_ eq 'whatever' } @array;

(List::MoreUtils is on CPAN)