Jobius,
    I get this and similiar messages whenever I try to do something silly like accessing the 5th element of a hash (which only happened once). Other times are when I try to use a variable that has gone out of scope or a typo'd variable name. Why don't you post the code and the full output (USE README TAGS) so that we can take a better look at it.
 
Thanks,
Kristofer.