The syntax it is expecting (I think) is:

open($article, ">>", indexer.txt") or die $!;

$nntp->article($last, $article);
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Note the use of a scalar file handle ($fh instead of FH). Also note the few modernisations I gave your code.

Your syntax (<FH>) would use the <> operator to try to read from the file handle and pass the result as an argument, rather than pass the actual file handle.