Why not just

c:\@Work\Perl\monks>perl -wMstrict -le
"my @test_data = (
   'a',
   '  b',
   '  e',
   '    f',
   '      g',
   '  c',
   '    d',
   'h',
   '  i',
   '    j',
   );
 print for @test_data;
 print '===========';
 ;;
 print $_->[0] for
   sort { $a->[1] cmp $b->[1] }
   map [ $_, m{ \w+ }xmsg ],
   @test_data
   ;
"
a
  b
  e
    f
      g
  c
    d
h
  i
    j
===========
a
  b
  c
    d
  e
    f
      g
h
  i
    j
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