I thought I would comment because not one of the solutions given handle a reference to a reference. So here is my non-recursive solution.

I forgot to handle references to references, will be fixed in a few minutes
I also admit i forgot to test if it's defined, so I'll change that right away.
Thanks!

2;0 juerd@ouranos:~$ perl -e'undef christmas'
Segmentation fault
2;139 juerd@ouranos:~$
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