++, nice idea.  Here's a variation of your approach, making use of the postprocess option:

#!/usr/bin/perl -l
use File::Find;

my @dirs; 
find(
    {   wanted      => sub {},
        postprocess => sub {
            push @dirs, $File::Find::dir
              if index $dirs[-1]||"", $File::Find::dir;
        },
    },
    '/tmp/a'
);

print for @dirs;
[download]