...
# populate your %project
readItem(\%project);
...
sub readItem {
   my ($project) = @_;

   my $sub_project = $project->{...}->{...};
   etc.
}

##</code><code>##

my %multi_level = (
      'foo'  => { 1 => 'A', 2 => 'B' },
      'bar'  => { 'C' => 'see', 'T' => 'tea' },
);

my $sub_part = $multi_level{foo}; # sub_part is a hashref, 
# also pointed at by $multi_level{foo}
print $multi_level{foo}->{1} . "\n";  # works (prints 'A')

$sub_part->{3} = 'C';
print $multi_level{foo}->{3} . "\n"; # works (prints 'C')

my $part_two = $multi_level{foo}->{1}; # part_two is a 
# scalar, a copy of the value of $multi_level{foo}->{1}, 
# namely 'A'. Changing it will not change %multi_level

##</code><code>##

my $a = 'blah';
my $b = $a;
$b = 'foo';
print "$a\n";  # prints 'blah'
print "$b\n";  # prints 'foo'