You could convert all that logic to:

  my %mimeTypes = (
    'pdf' => 'application/pdf',
    'xls' => 'application/vnd.ms-excel',
    'xml' => 'text/xml',
    'csv' => 'text/plain');
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Then simply key into the hash with $format. I also suggest not using binmode STDOUT when the MIME type is text/plain.