use strict;
use warnings;

my @levels = '.';
while (<DATA>) {
    chomp;
    my ($prefix, $node) = split(/(?<=-- )/, $_, 2);
    my $level = length($prefix)/2-1;
    $levels[$level] = "$levels[$level-1]/$node";
    print("$prefix$levels[$level]\n");
}

__DATA__
|-- DIR_COLORS
|-- X11
| |-- WindowMaker
| | |-- WMGLOBAL
| | `-- WindowMaker
| |-- XF86Config
| |-- Xresources
| |-- Xsession
| |-- app-defaults
| | |-- Beforelight
| | |-- Bitmap
| | `-- Xvidtune
| |-- glx.conf
| |-- lbxproxy
| | `-- AtomControl
|-- xinetd.conf
|-- xinetd.d
| |-- cups-lpd
| |-- cvs
| |-- fam
| |-- proftpd-xinetd
| |-- rsync
| `-- sshd-xinetd
|-- xml
| `-- catalog
`-- xpdfrc
[download]

|-- ./DIR_COLORS
|-- ./X11
| |-- ./X11/WindowMaker
| | |-- ./X11/WindowMaker/WMGLOBAL
| | `-- ./X11/WindowMaker/WindowMaker
| |-- ./X11/XF86Config
| |-- ./X11/Xresources
| |-- ./X11/Xsession
| |-- ./X11/app-defaults
| | |-- ./X11/app-defaults/Beforelight
| | |-- ./X11/app-defaults/Bitmap
| | `-- ./X11/app-defaults/Xvidtune
| |-- ./X11/glx.conf
| |-- ./X11/lbxproxy
| | `-- ./X11/lbxproxy/AtomControl
|-- ./xinetd.conf
|-- ./xinetd.d
| |-- ./xinetd.d/cups-lpd
| |-- ./xinetd.d/cvs
| |-- ./xinetd.d/fam
| |-- ./xinetd.d/proftpd-xinetd
| |-- ./xinetd.d/rsync
| `-- ./xinetd.d/sshd-xinetd
|-- ./xml
| `-- ./xml/catalog
`-- ./xpdfrc
[download]

I would think
print("$levels[$level]\n");
would make more sense than
print("$prefix$levels[$level]\n");
but I gave you what you asked for.