$hash{20}      = 1;  # Same as $hash{'20'}  = 1;
$hash{2e1}     = 2;  # Same as $hash{'20'}  = 2;
$hash{'2e1'}   = 3;  # Same as $hash{'2e1'} = 3;
$hash{'2E1'}   = 4;  # Same as $hash{'2E1'} = 4;
$hash{0+'2e1'} = 5;  # Same as $hash{'20'}  = 5;
$hash{0+'2E1'} = 6;  # Same as $hash{'20'}  = 6;

# '2E1' => 4, '2e1' => 3, '20' => 6