#!/usr/bin/perl
use strict;
use warnings;

    my %dtree = (
        L1 => {
            L2b => {L2b1 => {}, L2b2 => {}},
            L2a => {L3b => {L4a => {}}, L3a => {}}
            },
        L3b => {L4a  => {}},
        L2b => {L2b1 => {}, L2b2 => {}},
        L2a => {L3b  => {L4a => {}}, L3a => {}}
        );

processDtree(\%dtree);


sub processDtree {
    my ($rh_dtree, @nodes) = @_;

    print join ('.', map {"nested-$_"} @nodes), "\n" if @nodes;
    processDtree ($rh_dtree->{$_}, @nodes, $_) for sort keys %$rh_dtree;
}