My guess is that this code comes straight from your script, somewhere near the back. Am I right? In that case, you're probably trying to run objFood->new before @objFood::ISA is even set.

Try assigning to @ISA in a BEGIN block.

BEGIN { @objFood::ISA = qw(Object); }
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The reason why you don't have to do that when the classes are in their own module, is the fact that the bare code in a module is run as soon as the file is compiled, through use.