exists $is_prime{$_} and $is_prime{$_}

##</code><code>##

#!/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__