Why does testing for definition of a value create a key?

Bad question. It's not defined that creates it. 

>perl -MData::Dumper -e"grep { 0 } @h{qw(a b)}; print Dumper \%h;"
$VAR1 = {
          'a' => undef,
          'b' => undef
        };

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grep aliases $_ to the element being tested. That means is needs something to alias to. That causes the element to be created.

Is there a way around the key creation?

Yes, pass the keys to grep, not the values. 

my @defined_values = map $benchmarks{$_},
                     grep defined($benchmarks{$_}),
                     @BASELINES;

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or maybe you really want 

my @defined_values = map $benchmarks{$_},
                     grep exists($benchmarks{$_}),
                     @BASELINES;

[download]

In reply to Re: Accidentally creating hash elements with grep { defined } on a hash slice by ikegami
in thread Accidentally creating hash elements with grep { defined } on a hash slice by ferment

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