#!/usr/bin/perl
use warnings;
use strict;

my @castaways = qw( john Jane LUKE );
my @list_1 = qw( not John JANE luke );

my %hash = map { lc, $_ } @castaways;
for my $person (@list_1) {
    if (exists $hash{ lc $person }) {
        print "$person is a castaway (as $hash{ lc $person }).\n";
    }
}