Or just:

my @parents = qw( a b c );
my @children = qw( 1 2 3 4 );
my %abc :shared;

%{ $abc{ $_ } //= &share({}) } = map{ $_ => 1 } @children for @parents
+;

pp \%abc;
[download]

Which prints:

do {
  my $a = {
    # tied threads::shared::tie
    a => {
           # tied threads::shared::tie
           1 => 1,
           2 => 1,
           3 => 'fix',
           4 => 'fix',
         },
    b => {
           # tied threads::shared::tie
           1 => 1,
           2 => 1,
           3 => 'fix',
           4 => 'fix',
         },
    c => {
           # tied threads::shared::tie
           1 => 1,
           2 => 1,
           3 => 'fix',
           4 => 'fix',
         },
  };
  $a->{a}{3} = $a->{a}{1};
  $a->{a}{4} = $a->{a}{2};
  $a->{b}{3} = $a->{b}{1};
  $a->{b}{4} = $a->{b}{2};
  $a->{c}{3} = $a->{c}{1};
  $a->{c}{4} = $a->{c}{2};
  $a;
}
[download]