The following code works as expected:

use warnings;
use strict;
use Data::Dumper;

my %HoH = (
  first => {1 => 'First 1', 2 => 'First 2'},
  second => {1 => 'second ', 2 => 'second 2'});

print "Original hash: " . Dumper (\%HoH);

my %newHoH = %{inAndOut (%HoH)};

print "\nnew hash: " . Dumper (\%newHoH);


sub inAndOut {
  my (%hash) = @_;
  
  print "\nin sub: " . Dumper (\%hash);
  return {%hash};
}
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Original hash: $VAR1 = {
          'first' => {
                       '1' => 'First 1',
                       '2' => 'First 2'
                     },
          'second' => {
                        '1' => 'second ',
                        '2' => 'second 2'
                      }
        };

in sub: $VAR1 = {
          'first' => {
                       '1' => 'First 1',
                       '2' => 'First 2'
                     },
          'second' => {
                        '1' => 'second ',
                        '2' => 'second 2'
                      }
        };

new hash: $VAR1 = {
          'first' => {
                       '1' => 'First 1',
                       '2' => 'First 2'
                     },
          'second' => {
                        '1' => 'second ',
                        '2' => 'second 2'
                      }
        };
[download]

but the equivelent of your code:

use warnings;
use strict;
use Data::Dumper;

my %HoH = (
  first => {1 => 'First 1', 2 => 'First 2'},
  second => {1 => 'second ', 2 => 'second 2'});

my %bogusHoH = %{bogusInAndOut (%HoH)};

print "\nbogus hash: " . Dumper (\%bogusHoH);


sub bogusInAndOut {
  my (%hash) = @_;
  return %hash;
}
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generates warning:

Can't use string ("2/8") as a HASH ref while "strict refs" in use at C
+:\Documents and Settings\Peter.WINDOMAIN\My Documents\PerlMonks\nonam
+e.pl line 9.
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You would do well to use strict; use warnings; in your code to catch this sort of thing earlier for yourself.