use isnum approach for checking if Compute() is for a string or a number

[dpvc]

See the last answer here

Basically,

sub isnum ($) {
    return 0 if $_[0] eq '';
    $_[0] ^ $_[0] ? 0 : 1
}

This would make it possible to handle exponential notation better if a number is close to zero, for example. E.g., Compute(1.23e-5).