Here's one that lets the regex engine find the largest square all by itself :) 

#!/usr/bin/perl

# https://perlmonks.org/?node_id=1233341

use strict;
use warnings;

my $original = local $_ = do { local $/; <DATA> };
s/.\K  //g;                       # tweaks to make it easier to work w
+ith
my $width = /\n/ && $-[0];
s/.+/sprintf "%-${width}s", $&/ge;
my @squares;

while( s/x| / / )
  {
  my $pos = $-[0];
  pos($_) = $pos;
  /\G( +)(??{ ('.{' . ($width + 1 - length $1) . "}$1") x ~-length $1}
+)/s
    or die "tricky regex failed - oops";
  my $size = $& =~ tr/\n// + 1;
  for my $n ( 1 .. $size )
    {
    substr $_, $pos, $size, ('A' .. 'Z')[@squares % 26] x $size;
    $pos += $width + 1;
    }
  push @squares, [ $pos % ($width + 1), int $pos / ($width + 1), $size
+ ];
  }

use Data::Dump 'dd'; dd \@squares;
print $original;
print s/./$&  /gr =~ s/  $//gmr;

__DATA__
0  1  2  3  4  5  6  7  8  9  0  1  2
1
2                             x
3                    x
4        x
5  x
6              x
7                       x  x
8        x
9        x                 x
0        x
1     x                 x
2

[download]

EDIT: removed leftover unneeded instructions.

In reply to Re: Tricky math problem ... by tybalt89
in thread Tricky math problem ... by baxy77bax

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.