use strict;
use warnings;
use feature 'say';

my $hash_ref = {}; # SCALAR pointing to an empty HASH reference
my %hash     = (); # actual HASH 

$hash{key1}{key2}         = 'val1';
$hash_ref->{key1}->{key2} = 'val2';

$hash_ref = {           # HASH reference, note: curly brace ***
  key1 => {             # key1 is a nested HASH reference
    key2 => 'val2',     # key2 is pointing to the value "val2" 
  },
};

%hash = (               # actual HASH, note: parens ***
  key1 => {             # key1 is a nested HASH reference
    key2 => 'val1',     # key2 is pointing to the value "val2" 
  },
);

# accessing "key2" in $hash_ref:
say $hash_ref->{key1}->{key2}; # the first "->" is important 
say $hash_ref->{key1}{key2}; 

# accessing "key2" in %hash:
say $hash{key1}{key2};     # lack of the first "->" is important
say $hash{key1}->{key2};

__END__