So, are my tests flawed
Kinda. Your benchmarks are dominated by the time taken to generate @a and 'map log @a'. If these are precomputed you can start to see a big improvement with slicing and a smaller improvement with pre-sizing. 
use strict;
use warnings;
use Benchmark qw/ cmpthese /;;

for my $count ( 100, 1_000, 10_000, 100_000 ) {
    my @a = map { log } 2 .. $count;
    my @b = map log @a;
    cmpthese( -5, {
        1 => sub {
            my %h;
            keys %h = @a;
            $h{ $_ } = log for @a;
            return \%h
        },
        2 => sub {
            my %h;
            $h{ $_ } = log for @a;
            return \%h
        },
        3 => sub {
            my %h;
            keys %h = @a;
            @h{ @a } = @b;
            return \%h
        },
        4 => sub {
            my %h;
            @h{ @a } = @b;
            return \%h
        },
    })
}

$ perl5240o ~/tmp/x.pl 
     Rate    2    1    4    3
2 14381/s   --  -1% -25% -26%
1 14516/s   1%   -- -24% -26%
4 19198/s  33%  32%   --  -2%
3 19510/s  36%  34%   2%   --
    Rate    2    1    4    3
2 1427/s   --  -4% -25% -29%
1 1480/s   4%   -- -22% -26%
4 1895/s  33%  28%   --  -5%
3 2004/s  40%  35%   6%   --
   Rate    2    1    4    3
2 131/s   --  -6% -22% -29%
1 140/s   6%   -- -17% -25%
4 168/s  28%  20%   --  -9%
3 185/s  41%  33%  10%   --
    Rate    2    1    4    3
2 6.50/s   --  -6% -12% -18%
1 6.94/s   7%   --  -6% -12%
4 7.39/s  14%   6%   --  -7%
3 7.91/s  22%  14%   7%   --
$

[download]

Dave.

In reply to Re: Does "preallocating hash improve performance"? Or "using a hash slice"? by dave_the_m
in thread Does "preallocating hash improve performance"? Or "using a hash slice"? by vr

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