Compute() actually produce what you are looking for. They others may not produce an error message, but they don't compute the correct value. That is because e is not a predefined value in Perl, and so it is treated as an unquoted string. In some situations, that string is converted to a number by turning it into a zero, but in other cases, it causes a syntax error. But even when it doesn't the answer you get won't be what you expect.
Only inside a Compute() string is e defined as you would expect. We will talk more about these issues on Monday.
In perl code, you could do
$e = exp(1); $a = ($k1*$d*$e);for example. There is also a constant
$E defined by PG that you could use (if you don't mind the capitalization difference).