GrandFather has answered the question, so this just another data point.

Text::CSV returns undef when it is passed incorrect arguments. If you had added a print for the $csv as well as $fh then this would have come up as a warning about printing an undefined value. Or you could have added:

die "CSV object is undefined" if !defined $csv;
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This is one of those things that is much clearer in hindsight.