"What is your opinion?"
Unfortunately, you've provided very little information upon which an opinion might be based.
Answers to the following would have been useful in your OP.
How did you declare %hash? my? our? no strict 'vars'; and just let it pop into existence? something else?
In what context are you using %hash?
Why are you attempting to get rid of its contents (with undef or =())?
Where's your benchmark code?
Where's your benchmark results?
[Sorry, but just saying "%hash = () is slower than undef %hash about 1.2 times" is not sufficient.]
Given all those unknowns, my opinion is that you should consider code like this:
my %hash = (...);
# operations on %hash here
# %hash doesn't exist here. Does that solve your problem?
Of course, with some answers to those questions, a better response might be forthcoming.
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