use strict;  
use warnings; 
my @arr; 
my %hash = ('a0' => {'b0' => 'c0',}, 
            'a1' => 'b1', 
            'a2' => {'b2' => {'c2' => 'd2'},}, 
            ); 
 
my $hash_ref = \%hash; 
 
foreach my $ke (sort(keys(%hash))){ 
       func($hash_ref,$ke); 
    } 
 
sub func { 
    my $ref =shift; 
    my $ke = shift; 
     
    if (ref($ref->{$ke}) =~ /HASH/){ 
        push(@arr,$ke); 
             foreach my $ke_2 (sort(keys(%{$ref->{$ke}}))){ 
             func(\%{$ref->{$ke}},$ke_2); 
         } 
  
     } 
     else{ 
         push (@arr,$ke); 
         push (@arr,$ref->{$ke}); 
         push (@arr,"END"); 
         return; 
     } 
} 
 
print "@arr";

use strict; 
use warnings; 
use Readonly; 
Readonly::hash my %hash => ( 
    A => { 
        B => 'value_AB', 
    }, 
    C => { 
        D => { 
            E => 'value_CDE', 
        }, 
    }, 
);  
 
while (my $line = <DATA>) { 
    my $value; 
    if ( $line =~ /[^:]([A-Z]):([A-Z])[^:]/) { 
        $value = $hash{$1}{$2}; 
    } 
    elsif ( $line =~ /[^:]([A-Z]):([A-Z]):([A-Z])[^:]/ ) { 
        $value = $hash{$1}{$2}{$3}; 
    } 
    print $value, "\n"; 
} 
__DATA__ 
. HashElements- A:B  
2. HashElements- C:D:E