I get the same results. I'm using ActivePerl v5.6.1 on Win2k on a machine with little memory available. From the results below, I'm guessing you need less than 100MB free.

Hash 1 Rows/Sec: 62500.1
Hash 2 Rows/Sec: 4032.3
[download]

But the undef appears to be working nonetheless. I added a <STDIN> at the very top of the program, before the undef and at the very end of the program, and noted the size of perl's process using Task Manager before pressing Enter:

Start of execution:  <2MB
After one hash:     123MB
After two hashes:   131MB
[download]

Update: On a machine with more memory:

ActivePerl v5.8.0, WinXP, 640MB available

Hash 1 Rows/Sec: 250000.3
Hash 2 Rows/Sec: 250000.3

 <2MB
154MB
157MB


ActivePerl v5.6.1, WinXP, 640MB available

Hash 1 Rows/Sec: 200000.2
Hash 2 Rows/Sec: 200000.2

 <2MB
155MB
157MB
[download]

The slowdown is definitely related to virtual memory usage, but I don't know why it only affects the second pass.