#!/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", ["xray", "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;
  }