Please help enlighten me on the behaviour of "%$hash{'output'}" in the snippet of code below

my %hash = (
  'left_fills' => 'Name: a7',
  'main_title' => 'Main Area'
);

# call do_output
# Simplifed to pass just one hash value
do_output(
         output => \%hash,
);

sub do_output {
   my %hash = @_;
   my $output = $hash{'output'} || 0;

   # Dumped 1 (OK)
   print Dumper($output);

   # Dumped 2 (OK)
   print Dumper(%$output);

   # Dumped 3 (OK)
   print Dumper($hash{'output'});

   # Dumped 4 (Script died)
   print Dumper(%$hash{'output'});
}

# print Dumper($output);
$VAR1 = { 'left_fills' => 'Name: a7', 'main_title' => 'Main Area' };

# print Dumper(%$output);
$VAR1 = 'left_fills'; $VAR2 = 'Name: a7'; $VAR3 = 'main_title'; $VAR4 
+= 'Main Area';

# print Dumper($hash{'output'});
$VAR1 = { 'left_fills' => 'Name: a7', 'main_title' => 'Main Area' };

# print Dumper(%$hash{'output'});
Error: Global symbol "$hash" requires explicit package named at test.p
+l line...
[download]

When I did a "ref" of both "$output" and '$hash{'output'}", the result is HASH for both. I'm puzzled why their behaviour isn't exactly identical.

As usual, thanks in advance :)