$a = Real(0.6**20);
as this will make $a a MathObject, which will be handled better. As a Perl real, $a is converted to a string and then passed to Compute() to be converted to a MathObject, but the string version of this Perl real is 3.65616e-5 which MathObjects considers to be 3.65616 times e minus 5 rather than scientific notation (which would be 3.65616E-5).
Alternatively, you could use
Enter a: [_________]{uc($a)}
to force the lower-case "e" to an upper-case one.
Davide