Just because it looked like fun, and so far there are no examples of horizontal word trees.

#!/usr/bin/perl

use strict;
use warnings; # https://perlmonks.org/?node_id=998803

my $tree = [ [ [[[[["a", "b"], "c"], ["d", "e"]], [[["f", "g"], "h"], 
+[["i", "j"], ["k", ["l", "m"]]]]], ["n", [[["o", "p"], "q"], ["r", "s
+"]]]], ["t", ["u", "v"]] ], [["w", ["x", "y"]], "z"] ];

$tree = [ [ [[[[["alpha", "bravo"], "charlie"], ["delta", "echo"]], [[
+["foxtrot", "golf"], "hotel"], [["indigo", "juliet"], ["kilo", ["lima
+", "mike"]]]]], ["november", [[["oscar", "papa"], "quebec"], ["romeo"
+, "sierra"]]]], ["tango", ["uniform", "victor"]] ], [["whiskey", ["xr
+ay", "yankee"]], "zulu"] ];

print tree( $tree );

sub half { ' ' x ( length(shift) >> 1 ) }

sub tree
  {
  my $tree = shift;
  ref $tree or return "$tree\n" =~ s/  +/ /gr; # horizontal word
  ref $tree or return $tree =~ s/./$&\n/gr; # vertical word
  my ($padl, $padr) = map $_->[0] =~ tr// /cr, # pads
    my ($left, $right) = map [ tree($_) =~ /.+/g ], @$tree;
  use List::AllUtils qw( pairwise );
  local $_ = join '', pairwise # paste two blocks side by side
    { "@{[($a // $padl)]}  @{[($b // $padr)]}\n" } @$left, @$right;
  return s{^(?=( *)(\S.*?) {2,}(\S.*?)( *\n))}{
    my $span = ' ' x ($+[3] - $-[2] - 2 - length half($2) . half($3));
    "$1 " . half($2) . $span =~ tr/ /_/r . half($3) . " $4"
    . $1 . half($2) . "/$span\\" . half($3) . $4
    }er;
  }
[download]

Outputs:

                                                                      
+                                         ____________________________
+_______________________________________           
                                                                      
+                                        /                            
+                                       \          
                                                                      
+     ________________________________________________________________
+_______                          ______________   
                                                                      
+    /                                                                
+       \                        /              \  
                                         _____________________________
+______________________________________                               
+  ___________              __________         zulu
                                        /                             
+                                      \                              
+ /           \            /          \            
                     _______________________________________          
+                           _______________________                  t
+ango      _______      whiskey     _____          
                    /                                       \         
+                          /                       \                  
+         /       \                /     \         
             _______________                     _____________________
+__                    november             _______________           
+      uniform  victor           xray  yankee      
            /               \                   /                     
+  \                                       /               \          
+                                                  
       __________         _____             _________             ____
+__________                            _________         ______       
+                                                  
      /          \       /     \           /         \           /    
+          \                          /         \       /      \      
+                                                  
   ______     charlie  delta  echo      ______     hotel      ______  
+       _______                     _____     quebec  romeo  sierra   
+                                                  
  /      \                             /      \              /      \ 
+      /       \                   /     \                            
+                                                  
alpha  bravo                        foxtrot  golf         indigo  juli
+et  kilo     ____               oscar  papa                          
+                                                  
                                                                      
+            /    \                                                   
+                                                  
                                                                      
+          lima  mike
[download]

By commenting out the "horizontal word" line it produces:

                                              ________________________
+_     
                                             /                        
+ \    
                               ____________________________          _
+_____ 
                              /                            \        / 
+     \
                 ___________________________              ___      ___
+     z
                /                           \            /   \    /   
+\    u
        ________________                _________        t   __   w   
+__   l
       /                \              /         \       a  /  \  h  /
+  \  u
     _____          _________          n       _____     n  u  v  i  x
+  y   
    /     \        /         \         o      /     \    g  n  i  s  r
+  a   
   ___    __      ___      _____       v     ___    __   o  i  c  k  a
+  n   
  /   \  /  \    /   \    /     \      e    /   \  /  \     f  t  e  y
+  k   
 __   c  d  e   __   h   __    ___     m   __   q  r  s     o  o  y   
+  e   
/  \  h  e  c  /  \  o  /  \  /   \    b  /  \  u  o  i     r  r      
+  e   
a  b  a  l  h  f  g  t  i  j  k   __   e  o  p  e  m  e     m         
+      
l  r  r  t  o  o  o  e  n  u  i  /  \  r  s  a  b  e  r               
+      
p  a  l  a     x  l  l  d  l  l  l  m     c  p  e  o  r               
+      
h  v  i        t  f     i  i  o  i  i     a  a  c     a               
+      
a  o  e        r        g  e     m  k     r                           
+      
               o        o  t     a  e                                 
+      
               t
[download]