%singleLevelHash = (
 'thirdLevelSampleKeyOne'   => 'thirdLevelSampleValue',
 'thirdLevelSampleKeyTwo'   => 'thirdLevelSampleValue',
 'thirdLevelSampleKeyThree' => 'thirdLevelSampleValue',
);

##</code><code>##

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my %singleLevelHash = ();

my %multiLevelHash = (
    'firstSampleKey' => 'SampleValue',
    'secondSampleKey' => {
	'secondLevelSampleKey' => { 
	    'thirdLevelSampleKeyOne'   => 'thirdLevelSampleValue',
	    'thirdLevelSampleKeyTwo'   => 'thirdLevelSampleValue',
	    'thirdLevelSampleKeyThree' => 'thirdLevelSampleValue',
	}
    }
);

print Dumper \%multiLevelHash;

foreach my $firstSampleKey ( sort keys %multiLevelHash ) {
    print $firstSampleKey . "\n";
    foreach my $secondLevelSampleKey ( sort keys $multiLevelHash{$firstSampleKey} ) {
	print $secondLevelSampleKey . "\n";
    }
}

__DATA__

$VAR1 = {
          'firstSampleKey' => 'SampleValue',
          'secondSampleKey' => {
                                 'secondLevelSampleKey' => {
                                                             'thirdLevelSampleKeyOne' => 'thirdLevelSampleValue',
                                                             'thirdLevelSampleKeyThree' => 'thirdLevelSampleValue',
                                                             'thirdLevelSampleKeyTwo' => 'thirdLevelSampleValue'
                                                           }
                               }
        };

firstSampleKey
Type of argument to keys on reference must be unblessed hashref or arrayref at hash.pl line 21.