What decides the order of keys when I print a Perl hash?

Question

activePerl 5.8 based

#!C:\Perl\bin\perl.exe
use strict;
use warnings;

# declare a new hash
my %some_hash;

%some_hash = ("foo", 35, "bar", 12.4, 2.5, "hello",
      "wilma", 1.72e30, "betty", "bye\n");

my @any_array;
@any_array = %some_hash;

print %some_hash;
print "\n";
print @any_array;
print "\n";
print $any_array[0];
print "\n";
print $any_array[1];
print "\n";
print $any_array[2];
print "\n";
print $any_array[3];
print "\n";
print $any_array[4];
print "\n";
print $any_array[5];
print "\n";
print $any_array[6];
print "\n";
print $any_array[7];
print "\n";
print $any_array[8];
print "\n";
print $any_array[9];

Output as this

D:\learning\perl>test.pl
bettybye
bar12.4wilma1.72e+030foo352.5hello
bettybye
bar12.4wilma1.72e+030foo352.5hello
betty
bye

bar
12.4
wilma
1.72e+030
foo
35
2.5
hello
D:\learning\perl>

What decided the elements print order in my sample code?

Any rule to follow when print a mixed(strings, numbers) hash in Perl? Thank you.

bar12.4wilma1.72e+030foo352.5hello

[Updated]

With you guys help, i updated the code as below.

#!C:\Perl\bin\perl.exe
use strict;
use warnings;

# declare a new hash
my %some_hash;

%some_hash = ("foo", 35, "bar", 12.4, 2.5, "hello",
      "wilma", 1.72e30, "betty", "bye");

my @any_array;
@any_array = %some_hash;

print %some_hash;
print "\n";
print "\n";
print @any_array;
print "\n";
print "\n";

my @keys;
@keys = keys %some_hash;
for my $k (sort @keys)
{
    print $k, $some_hash{$k};
}

output

D:\learning\perl>test.pl
bettybyebar12.4wilma1.72e+030foo352.5hello

bettybyebar12.4wilma1.72e+030foo352.5hello

2.5hellobar12.4bettybyefoo35wilma1.72e+030
D:\learning\perl>

Finially, after called keys and sort functions. The hash keys print followed the rule below

2.5hellobar12.4bettybyefoo35wilma1.72e+030

Answer 1

Elements of a hash are printed out in their internal order, which can not be relied upon and will change as elements are added and removed. If you need all of the elements of a hash in some sort of order, sort the keys, and use that list to index the hash.

If you are looking for a structure that holds its elements in order, either use an array, or use one of the ordered hash's on CPAN.

the only ordering you can rely upon from a list context hash expansion is that key => value pairs will be together.

Answer 2

From perldoc -f keys:

The keys of a hash are returned in an apparently random order. The actual random order is subject to change in future versions of Perl, but it is guaranteed to be the same order as either the values or each function produces (given that the hash has not been modified). Since Perl 5.8.1 the ordering is different even between different runs of Perl for security reasons (see Algorithmic Complexity Attacks in perlsec).

...

Perl has never guaranteed any ordering of the hash keys, and the ordering has already changed several times during the lifetime of Perl 5. Also, the ordering of hash keys has always been, and continues to be, affected by the insertion order.

Also note that while the order of the hash elements might be randomised, this "pseudoordering" should not be used for applications like shuffling a list randomly (use List::Util::shuffle() for that, see List::Util, a standard core module since Perl 5.8.0; or the CPAN module Algorithm::Numerical::Shuffle), or for generating permutations (use e.g. the CPAN modules Algorithm::Permute or Algorithm::FastPermute), or for any cryptographic applications.

---

Note: since you are evaluating a hash in list context, you are at least guaranteed that each key is followed by its corresponding value; e.g. you will never see an output of a 4 b 3 c 2 d 1.