A similar approach to moritz's but using a map instead of a while loop to take advantage of the default behaviour of length to operate on $_.

$ perl -E '
> @strs = qw{
>    xxxxxxxNNNNNxxxxxNxxxxNNNNNNNNxxx
>    xxNxxxNNNNNNNNNxxxxxx
>    };
> foreach $str ( @strs )
> {
>     print qq{$str : };
>     say join q{, },
>        map length, $str =~ m{(N+)}g;
> }'
xxxxxxxNNNNNxxxxxNxxxxNNNNNNNNxxx : 5, 1, 8
xxNxxxNNNNNNNNNxxxxxx : 1, 9
$
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I hope this is useful.

Update: Changed second print statement to a say and got rid of the say with no arguments. What was I thinking?