No strict and no warnings means that you are not alerted to the mistake of passing the bareword names to which_element_is().

use strict;
use warnings;

my @array = qw /fred barney betty dino wilma pebbles bamm-bamn/;
my $result = which_element_is ("dino", @array);

sub which_element_is {
    my ($what, @array) = @_;
    foreach (0 .. $#array) {
        if ($what eq $array[$_]) {
            print "$_\n";
            return $_;

        }
    }
    print "99\n";
    -1;
}

print "Item $result is $array[$result]\n"
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Always use strict and warnings. Always.