The CGI.pm documentation tells us that the filename returned doubles as a file handle, so you can directly read from that parameter. This is some magic that best works when not looked closely at :). The documentation then goes on recommending the upload() function, available since version 2.47 :

$fh = $query->upload('uploaded_file');
while (<$fh>) {
  print;
}
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Using upload() gives you the additional advantage of returning an array of filehandles if there are many upload fields with the same name.