!/usr/bin/perl -w -p 
s/^(\*+)/ 
$n = length($1), 
$n < $o ? @l[$n+1..$#l] = (0)x($#l-$n) : 0, 
$l[$n]++, 
$o = $n, 
join(".",@l[1..$n]) 
/e;
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The item on the right of an s///e is a full block, so you don't need to haggle it all into a single expression. Thus, using a more natural coding style, this would be:

!/usr/bin/perl -w -p 
s {
  ^(\*+) # look for stars at the beginning of line
}{
  my $n = length($1); # get number of stars
  @l[$n+1..$#l] = (0)x($#l-$n) if $n < $o; # @l is global
  $l[$n]++;
  $o = $n; # $o is global
  join(".",@l[1..$n]); # returning this
}ex;
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I see a possibility (I think) of having only one global rather than two, but I'm not quite ready to rewrite that yet... just wanted to show the isomorphic equivalent of your code in a cleaner syntax.

-- Randal L. Schwartz, Perl hacker