Fun little problem :)

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1213391

use strict;
use warnings;

my @test_data = (
    'a',          #
    '  b',        #
    '  e',        #
    '    f',      #
    '      g',    #
    '  c',        #
    '    d',      #
    'h',          #
    '  i',        #
    '    j',      #
);

my $onestring = join '', map "$_\n", @test_data;
print $onestring, "=" x 60, "\n";

print group($onestring);

sub group
  {
  my @groups;
  push @groups, "$1" . group("$3") while $_[0] =~ /(( *).*\n)((?:\2 +.
+*\n)*)/g;
  join '', sort @groups;
  }
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