$$vars{'test'} is a visual pain for me because of the split millisecond where I ask myself about precedence. OTH the -> in $vars->{'test'} dates back to at least C where is used in exactly the same way and context.

Performance-wise I don't think there is a difference (OSX 10.13 and perl 5.28.3):

use Benchmark qw( cmpthese );

cmpthese(-5, {
        deref => 'use strict; use warnings; my $x = { map { $_ => rand
+ } 1..50 };
                  $x->{$_} = $_ for 1..100',
        none  => 'use strict; use warnings; my $x = { map { $_ => rand
+ } 1..50 };
                  $$x{$_} = $_ for 1..100'
});

         Rate deref  none
deref 12483/s    --   -1%
none  12611/s    1%    --

         Rate  none deref
none  11938/s    --   -0%
deref 11967/s    0%    --
[download]

Or,

use Benchmark qw( cmpthese );

cmpthese(-5, {
    deref => 'use strict; use warnings;
          srand 42;
          my $x = { map { rand() => rand() } 1..50 };
          srand 42;
          $x->{rand()} = rand() for 1..100',
    none  => 'use strict; use warnings;
          srand 42;
          my $x = { map { rand() => rand() } 1..50 };
          srand 42;
          $$x{rand()} = rand() for 1..100'
});
[download]

For benchmarks see also: https://stackoverflow.com/questions/18984323/how-expensive-is-it-to-dereference-an-array-ref-in-perl

bw, bliako