This style of dereferencing works for me:

use warnings;
use strict;

my $dbm;
$dbm->{'A'}->{'20150301'}=10; # First quote for A
$dbm->{'A'}->{'20150302'}=11; # First quote for A
$dbm->{'B'}->{'20150301'}=8; # First quote for A
$dbm->{'B'}->{'20150302'}=8.5; # First quote for A

my @k = keys %{ $dbm->{A} };
use Data::Dumper; print Dumper(\@k);

__END__

$VAR1 = [
          '20150302',
          '20150301'
        ];
[download]