Thanks for the code, I used it to benchmark Perl 5.10.0 on Linux: 
              s/iter    With undef Without undef
With undef      17.2            --          -26%
Without undef   12.8           34%            --

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which, although a bigger difference than yours, isn't as bad as what oxone reported. This was on: 
Linux 2.6.25 #6 SMP Tue Aug 5 17:42:15 SGT 2008 i686 GNU/Linux
This is perl, v5.10.0 built for i486-linux-gnu-thread-multi

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In reply to Re^2: Bug when undefining a large hash by ggvaidya
in thread Bug when undefining a large hash by oxone

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