As I feared (though it makes sense)

tied_hashes do it with list assignments (they are sequential) but not with ano-hash assignment (they are precompiled, hence collisions are optimized)

FWIW

use strict;
use warnings;

use Data::Dump qw/pp/;

package HoA;
require Tie::Hash;
our @ISA = qw(Tie::StdHash);

sub STORE {
  my ($this,$key,$value) =@_;
  push @{$this->{$key}}, $value;
}


package main;

my $data_str= do { local $/;<DATA>};


tie my %h, "HoA";

my $h=\%h;


%h = eval "$data_str";
pp $h;

%h = eval "%{ { $data_str } }";
pp $h;


__DATA__
(
   alpha => 1,
   alpha => 2
)
[download]

shows

{
  # tied HoA
  alpha => [1, 2],
}
{
  # tied HoA
  alpha => [2],
}
[download]