From first_index in List::MoreUtils:

Returns the index of the first element in LIST for which the criterion in BLOCK is true. Sets $_ for each item in LIST in turn:

Update:

use List::MoreUtils qw/first_index/;

my @array = ( 6, 5, 4, 3, 2, 1 );
my $first_4_idx = first_index { $_ == 4 } @array;
print $first_4_idx;
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