use Data::Dumper;

my %hash;
$hash{  0  } = 'numeric zero';
$hash{ '0' } = 'sigle quoted zero'; # over write above
$hash{ '0'} = 'escaped zero in sigle quotes';
$hash{ "0"} = 'escaped zero in double quotes';
$hash{  0 } = 'escaped zero in non quotes';
$hash{  A  } = 'Uppercase character A';
$hash{  a  } = 'lowercase character A';
$hash{ ''  } = 'empty string';
$hash{undef} = 'undefined value';

print Dumper(%hash);

$VAR1 = {
          '' => 'empty string',
          'A' => 'Uppercase character A',
          '　' => 'escaped zero in double quotes',
          'a' => 'lowercase character A',
          'undef' => 'undefined value',
          'SCALAR(0x182fae4)' => 'escaped zero in non quotes',
          '0' => 'sigle quoted zero',
          '0' => 'escaped zero in sigle quotes'
        };