I'm not sure whether you're making your case based on cargo cultism or micro-optimization; neither is a good idea. Abusing undef benchmarks at about 20% faster than assigning empty list, but it relies on undocumented behavior.

You can get within 2% by saying 

@hash{@list} x= 0;

[download]
which is still odd (and gives a warning for using uninitialized values), but is at least documented, if your code is bottlenecking at your key-creation section. In the absence of a bottleneck, it is better to write clear code than marginally faster code.
Caution: Contents may have been coded under pressure.

In reply to Re^4: Adding Unique Elements to Array by Roy Johnson
in thread Adding Unique Elements to Array by thekestrel

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