Presuming you don't have more than 255 (or 16k for you Unicodians) levels in any one outline you could always convert into a character string and just sort on that (sort of like a radix sort).

use List::Util qw( max );
my @raw = qw( 1.3 2.3 2.1 2.0 2.0.3 1.2 2.0.2 1.0 );
my $longest = max map { my @c = split(/\./, $_); scalar @c } @raw; 

my @sorted =
  map { $_->[1] } sort { $a->[0] cmp $b->[0] } 
    map { my @c = split( /\./, +$_ ); 
          push @c, ("0") x $longest - @c;
          [ pack( "c*", @c ), $_ ] } @raw;

print join( "\n", @sorted ), "\n";
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Update; Dang, japhy beat me. And the filling out to the same length's unnecessary; I don't know why I thought I needed to do that. Meh, need more caffeine.