Here is some code to show the difference (perlvar as you quoted gives a good description):

use strict;
use warnings;

my $plus;
my $N;

my $text = "abc123def";
my $text_match = qr/((\D+)(\d+))(?{ $plus = $+;$N = $^N })(\D+)/;
$text =~ $text_match;
print "$plus, $N\n";

$text_match = qr/(\D+)(\d+)(?{ $plus = $+;$N = $^N })(\D+)/;
$text =~ $text_match;
print "$plus, $N\n";
[download]

update:oh yeah the results:

__END__
123, abc123
123, 123
[download]

-enlil