Calling defined for a hash key doesn't create anything.

There is a subtle difference internally between exists and defined. With exists, all Perl has to do is see if there's an entry in the hash table for that item. With defined, that value has to be inspected:

Benchmark: timing 1000000 iterations of defined, exists...
   defined: 21 wallclock secs (21.36 usr +  0.00 sys = 21.36 CPU)
    exists: 19 wallclock secs (18.45 usr +  0.00 sys = 18.45 CPU)
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Using code like this:

# %hash is a hash with values for one/two/tre/for

sub t_exists {
    1 if exists $hash{one};
    1 if exists $hash{two};
    1 if exists $hash{tre};
    1 if exists $hash{for};
    1 if exists $hash{xxx};
    1 if exists $hash{xxy};
    1 if exists $hash{xxz};
    1 if exists $hash{xyx};
}
...
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Granted, that's a million iterations, and the difference is only a couple of seconds. Hardly worth arguing over, but it's just a difference in the way of thinking about hashes.. In my head, what I'm looking for is the presence of a certain key in the hash (exists), not whether or not the value for that key in the hash is defined. You're right that functionally they're equivalent in this case.