Here are some of the problems that need to be resolved.
First, you are trying to compare strings using "==". In Perl that needs to be "eq" instead.
Next, you are using the old style fun_cmp with the MathObjects version of the "tolerance" argument. For fun_cmp it is just "tol".
Also, to put the PopUp menu in when it is part of a MultiAnswer object use $multians->ans_rule(), not $multians->menu().
This at least takes care of the warnings.