Use of uninitialized value in concatenation (.) or string at x.pl line 47.
Use of uninitialized value in concatenation (.) or string at x.pl line 47.
...

##</code><code>##

$w_columns{$w_col_ix} = "$w_columns{$w_col_ix}${w_pad}${w_char}";

##</code><code>##

$w_columns{$w_col_ix} .= "${w_pad}${w_char}";

##</code><code>##

use strict;
use warnings;

my $a_rows  = [qw[one two three four five six seven eight nine ten]];
my $npad    = 4;
my $justify = 1;

rows_to_cols($a_rows, $npad, $justify);


sub rows_to_cols {
    my ($x, $p, $j) = @_;
    my $m = 0; map { $m = length $_ if $m < length $_ } @$x;
    ($j||0) && map { $_ = sprintf "%*s", $m, $_ } @$x;
    @_ =  map { ($")x($p||2),$_ } @$x;
    map { map { print substr($_, 0, 1, "") || $" } @_; print $/ } 1..$m
}

##</code><code>##

sub rows_to_cols {
    my ($x, $p, $j) = @_;

    my $m = 0; map { $m = length $_ if $m < length $_ } @$x;
    ##
    # Assigns to $m the maximum length of any word

    ($j||0) && map { $_ = sprintf "%*s", $m, $_ } @$x;
    ##
    # If bottom-justifying, converts every word to the
    # right-justified version of the same.  Thus:
    #
    #    [                       [
    #     'one',                  '  one',
    #     'two',                  '  two',
    #     'three',                'three',
    #     'four',                 ' four',
    #     'five',  => becomes =>  ' five',
    #     'six',                  '  six',
    #     'seven',                'seven',
    #     'eight',                'eight',
    #     'nine',                 ' nine',
    #     'ten'                   '  ten',
    #    ]                       ]              

    @_ =  map { ($")x($p||2),$_ } @$x;
    ##
    # Creates (in @_) vertical padding between each word.
    # For example:
    #  [
    #   ' ',
    #   ' ',
    #   ' ',
    #   ' ',
    #   '  one',
    #   ' ',
    #   ' ',
    #   ' ',
    #   ' ',
    #   '  two',
    #   ' ',
    #   ' ',
    #   ' ',
    #   ' ',
    #   'three',
    #   ' ',
    #   ' ',
    #   ...
    #   ' ',
    #   '  ten'
    #  ]
    # Note that the data is now in a horizontal format that,
    # if converted to the vertical equivalent, would be the
    # the desired result.

    map { map { print substr($_, 0, 1, "") || $" } @_; print $/ } 1..$m
    ##
    # This does the magic horizontal to vertical conversion
}