** Using WeBWorK **

I'm not sure why you don't just do something like this:

@NumberToName = ("zero","one","two","three","four","five");

$rh = random(1,5,1);                              # ratio count of urban mosquitoes
do { $rf = random(1,5,1); } until ($rf != $rh);   # ratio count of forest mosquitoes

$rht = $NumberToName[$rh];
$rft = $NumberToName[$rf];

There is certainly no need to recreate the hash for each of the two variables, and certainly no need to do it inside the do-until loop. There's really no need to a hash at all, as an array does just fine for this (since your hash keys are just small integers).