A small point: that s/// is pretty inefficient, and replacing it with:

  substr($str, 0, $pos1) = '';
[download]

makes it clear that you could do it implicitly in the previous line just by adding an extra argument:

  print $indent x $depth, substr( $str, 0, $pos1, '' );
[download]

dragonchild's refactored version would not need the $pos = $newpos + 2 correction if the (suitably renamed) $_print() did the same thing.

I'd be tempted to make it read a bit more cleanly by also passing in the depth to $_print() and include a slight reordering to flatten:

  my $_consume = sub {
    print $indent x $_[2], substr $_[0], 0, $_[1], '';
  }

  sub pp {
    return unless @_;
    my($str) = @_;
    my($length, $first) = f1($str);

    if (!defined $first) {
      $_consume->($str, $length, $depth);
    } elsif ($first eq CONST1) {
[download]

      $_consume->($str, $length, --$depth);

      $_consume->($str, $length + 1, --$depth);
    } elsif (defined(my $newlen = f2($str, $first))) {
      $_consume->($str, $newlen + 2, $depth);
    } else {
      $_consume->($str, $length, $depth++);
    }
    return $str;
  }
[download]

Update: passing $str to be modified in this way is probably not safe - if I remember right, any attached magic will cause a temporary copy to be passed instead - so some slight modifications to pass \$str instead are probably required.

Update 2: added a missing + 1

Hugo

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