Thank you all for your answers. I have checked our system, it showed: 
..... Thu Jul 10 00:12:43 UTC 2008 i686 GNU/Linux

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Does this mean I am using a 32 bits systm which only allow perl to use 4GB no matter how large my actual RAM is?

In reply to Re^2: out of memory problem by lightoverhead
in thread out of memory problem by lightoverhead

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