The difference is that $^N contains the last closed capture while $+ contains the last opened one. That is, $^N looks to right parens and $+ to left ones. That makes a difference for nested captures.

From the command line in 5.8.0:

$ perl -e'$_=foobar;/(oo(ba)r)/;print $+,$/'
ba
$ perl -e'$_=foobar;/(oo(ba)r)/;print $^N,$/'
oobar
$
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