#non shared hash version
my %abc;

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

for my $parent ( @parents )
{
  for my $child ( @children )
  {
      $abc{ $parent }{ $child } = 1;
  }
}


##</code><code>##

# shared hash version.
my %abc : shared ;

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

for my $parent ( @parents )
{
  unless (exists $abc{$parent})
  {
    my %p : shared;
    $abc{ $parent } = \%p;
  }
  
  for my $child ( @children )
  {
      unless ( exists $abc{$parent}{$child} )
      {
        my %c : shared;
        $abc{$parent}{$child}=\%c;
      }
      $abc{ $parent }{ $child } = 1;
  }
}