I used the built in perl debugger. Here is the result of that exercise.

M:\My Documents\Code\perl>perl -d test6.pl

Loading DB routines from perl5db.pl version 1.31
Editor support available.

Enter h or `h h' for help, or `perldoc perldebug' for more help.

main::(test6.pl:5):     my      $in_dir
=       "C:\\tmp";

  DB<1> l
5==>    my      $in_dir                                         =     
+  "C:\\tmp
";
6
7       sub select_files {
8:              print ("function was entered\n");
9       }
10
11:     $File::Find::prune                              =       0;
12:     unless (find(\&select_files, $in_dir)) {
13:             print ("Find failed with $!\n");
14      }

  DB<1> n
main::(test6.pl:11):    $File::Find::prune                            
+  =
0;

  DB<1> n
main::(test6.pl:12):    unless (find(\&select_files, $in_dir)) {

  DB<1> n
main::CODE(0x1e57a58)(C:/Perl/site/lib/Cwd.pm:752):
752:            $ENV{'PWD'} = Win32::GetCwd();
99 levels deep in subroutine calls!

  DB<1>
[download]

You will notice that I added some debugging code in an effort to figure out what is going on, but again none of it fired. Thanks.