Have you tried read()? If the records are fixed length you could read in for a length * number rows and then split on the newline or read in a big chunk and add logic to keep a partial record from the end of the buffer and add to it on the next read.

   while ( read($fh, $buf, $len) ) {
       @lines = split "\n", $buf;
       .
       .
       .
   }
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It may be faster, worth a try.