Just wanted to try this option out as well. So, I tried to use fc in the same example that choroba suggested and make it work. But it displays only "John" as a castaway.

#!/localperl/bin/perl

use warnings;
use strict;
use feature 'fc';

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

my %hash = map { $_, 1 } @castaways;
for my $person (@list_1) {
    if ( $hash{ fc($person) }) { # Tried with exists as well. Same res
+ult.
        print "$person is a castaway\n";
    }
}
[download]

Result: John is a castaway

So I think in my case, using lc is the only option. BTW I am using (v5.24.0)