you'll find perldsc an useful read for sure. But the task is not so complex: you can use some usful functions Perl offers to you. For exmple ref can tell you if a value you got fetching some key is a scalar or a hash ref. In the latter case you just to iterate over it as you've already done for the container hash.

use strict;
use warnings;

my %h1=('qwqw' => {'qw' => 'qww','df' => 'dfff'},);

my %hash=(
'AAA' => 'aaa',
'BBB' => 'bbb',
'CCC' => {'A1' => 'a1','B1' => 'b1','C1' => {'A2' => 'a2','B2' => 'b2'
+,'C2' => 'c2END'}},
'DDD' => 'ddd',
'EEE' => \%h1,
'FFF' => 'sdsfsds',
);


ddump (\%hash);

sub ddump
{
  my $ref  = shift;
  my $deep = shift||0;
  foreach my $k (sort keys %{$ref})
          {
            if (ref( ${$ref}{$k}))  {print "\t" x $deep."$k =>\n"; &dd
+ump (${$ref}{$k}, ($deep+1))}
            # key and value
            # else {print "\t" x ($deep)."$k => ${$ref}{$k}\n";}
            # or just the key as your need
            else {print "\t" x ($deep)."$k\n";}

          }

}


# OUTPUT
AAA
BBB
CCC =>
    A1
    B1
    C1 =>
        A2
        B2
        C2
DDD
EEE =>
    qwqw =>
        df
        qw
FFF
[download]

L*