function calculate_a_var()
{
   var calculated_var
   ......
   calculated_var = 1;
   ......
   return(calculated_var);
}
//
//
function test()
{
   ......
   var returned_var = 0;
   while(1)
   {
      if (on_mouse == 1)
      {
         returned_var = calculate_a_var();
      }
      if (returned_var == 1)
      {
         do something;
      }
   }
}

OR (with this calculate_a_var)

function test()
{
   ......
   while(1)
   {
      if ( (on_mouse == 1) && (calculate_a_var() == 1) )
      {
         do something;
      }
      wait(1);
   }
}