How are you determining that memory is being used?

Also, looking at your file1.pl, I think you probably want an exec here rather than using the back tick.

exec('/home/qiang/file2.pl', $storable_file, $base_dir);
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Because, from the docs, backticks does a fork and exec. So I would think you're probably doing fork from the ForkManager, Fork from backticks, then an exec from backticks. If that makes any sense.