Also, this might help you understand what is going on with
exists $is_prime{$_} and $is_prime{$_}
Run the code to see the differences between exists(), defined and true. See if you can understand the warnings this code gives.
#!/usr/bin/perl
use strict;
use warnings;
use feature qw/ say /;
use Data::Dumper;
$Data::Dumper::Sortkeys = 1;
my %hash;
$hash{'A'} = 1;
$hash{'B'} = 0;
$hash{'C'} = -1;
$hash{'D'} = undef;
say Data::Dumper->Dump([\%hash], ['*hash']);
for (qw/ A B C D E /) {
say "Key: $_ Val: $hash{$_}";
say exists $hash{$_} ? ' exists' : ' does not exist';
say defined $hash{$_} ? ' defined' : ' not defined';
say $hash{$_} ? ' true' : ' false';
say '- -' x 20;
}
__END__
The way forward always starts with a minimal test.