$ perl -MDevel::Peek -e'
   sub x {
      my %h;
      Dump(%h, 0);
      keys(%h) = 100;
      Dump(%h, 0);
      return \%h;
   }

   x() for 1..3;
' 2>&1 | grep MAX
  MAX = 7
  MAX = 127
  MAX = 7
  MAX = 127
  MAX = 7
  MAX = 127
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So all we have is a test that shows that @h{ @array } = @array; is faster than $h{ $_ } = $_ for @array;, but doesn't conclusively show any benefit from pre-allocating.