In Perl, there are 2 different kinds of hashes.

• A hash, which is defined by the % and ( ) characters - %hash = ( );
• A reference hash, which is defined with the $ and { } characters - $hash = { };

Let's say you define a hash array named %hash that contains a single key (foo). The foo key will contain an array of values. The [ ] characters are used to set the foo key as an array.

my %hash = ( 'foo' => [] );

The following can be used to print the number of keys in the hash.

my $keys   = keys %hash;
print "keys   = $keys \n";

Which should return the following.

keys = 1

Or if the hash contains two keys.

my %hash = ( 'foo' => [], 'bar' => [] );

The following should be returned.

keys = 2

Let's say the hash is multidimensional, where the foo key is the parent and the bar key is the child.

my %hash = ( 'foo' => [ { 'bar' => '' } ] );

The following should still be returned.

keys = 1

Here is how you would count the child keys below a parent key.

foreach my $foo (@{$hash{foo}}) {
  my $keys = keys %$foo;
  print "keys   = $keys \n";
}