Thanks to you both for your suggestions - I've done some experimenting and it seems that the $_ value inside the nested subroutine you can use in File::Find::Rule does contain the full path (so, full directory path + filename). Using the line...

my @results = File::Find::Rule->file->exec( sub { length($_[1]) < 255 
+} )->in( $dir );
[download]

...does indeed exclude any files with an invalid path length. Incidentally, these appear to have come about because directories are shared out half way down the tree and people have then written deep nested directories & files to it. When I come to analyse the directories from the top, the path length is now longer and over the 255 limit.

Cheers both!