Null value than rename file

I have a shell to a task but I should handle the null value in this. So in the task i cut the part of a word and i delete or rename the file, but need another if element for null. For example: I have word "pear" and I cut it all with this and the filename null: echo $fname | perl -p -e 's/:pear://g' I should rename now to "nullfile".