$ perl -Mstrict -Mwarnings -E '
    my %a=();

    $a{1}{"a"}{"A"}="FIRST";
    $a{1}{"c"}{"B"}="THIRD";
    $a{1}{"b"}{"C"}="SECOND";

    foreach my $k1 ( sort keys %a ) {
      foreach my $k2 ( sort keys %{$a{$k1}} ) {
        foreach my $k3 ( sort keys %{$a{$k1}{$k2}} ) {
         print "$a{$k1}{$k2}{$k3}\n";
        }
        
      }
    }
'
FIRST
SECOND
THIRD