Is there any drawback to adding the extra line.I can't think of one. Since I don't have perl 5.10.x running here (and don't have time to do a new installation at the moment), it is hard for me to test it out. There are some other places in the MathObjects library where strings are used as references, so you may run into this issue again as you work with some of the more modern problems that call on MathObjects directly. it is not yet clear to me exactly what behavior will be trapped.
If this is the only instance of the problem (and it seems to be the only obvious one), then I could work around it in a way that doesn't require turning off the strict refs. But for now, I think you are OK.
Davide