sort { $a =~ y/ //dr cmp $b =~ y/ //dr } @data

##</code><code>##

#!/usr/bin/env perl

use 5.014;
use warnings;

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

say_sorted(@$_) for @test_data;

sub say_sorted {
    my ($who, $data) = @_;
    say "Data from $who";
    say '-' x 20;
    say for @$data;
    say '-' x 20;
    say for sort { $a =~ y/ //dr cmp $b =~ y/ //dr } @$data;
    say '=' x 20;
}

##</code><code>##

Data from atcroft
--------------------
a
  b
  e
    f
      g
  c
    d
h
  i
    j
--------------------
a
  b
  c
    d
  e
    f
      g
h
  i
    j
====================
Data from tybalt
--------------------
j
  i
  h
    g
      f
  e
    d
c
  b
    a
--------------------
    a
  b
c
    d
  e
      f
    g
  h
  i
j
====================