If you want to find out both read and write, then you'll need to do 2 tests: one for read, one for write. Simply do a test with '-r' as well, such as the following (not tested, may/may not work as-is):

#!/usr/bin/perl -w
$|++;

use strict; 
use File::Find; 

my $dir = '/path/to/dir';
find(\&wanted, '/path/to/dir');

sub wanted { 
 if (not -w $_ && not -r $_) {
  print "Cannot read or write $File::Find::name.\n";
 } elsif (not -w) {
  print "Cannot write $File::Find::name.\n";
 } elsif (not -r) {
  print "Cannot read $File::Find::name.\n";
 }
}
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