I just tried this with Strawberryperl while a go, and remember it working just well. Now that I'm sitting on Windows 7* it doesn't seem to be able to get the list of files:

C:\Pics>dir
 Volume in drive C has no label.
 Volume Serial Number is 4C17-845B

 Directory of C:\Pics

20.02.2010  04:23    <DIR>          .
20.02.2010  04:23    <DIR>          ..
29.05.2009  14:59         1 578 868 DSCF3888.JPG
29.05.2009  14:59         1 565 825 DSCF3889.JPG
29.05.2009  14:59         1 643 616 DSCF3890.JPG
29.05.2009  14:59         1 664 020 DSCF3891.JPG
               4 File(s)      6 452 329 bytes
               2 Dir(s)  187 149 787 136 bytes free

C:\Pics>renamer -n -s DSCF okay *.JPG
Dry run, no changes were made.

C:\Pics>
[download]

Notice that I had to rename the script to renamer, but this does not affect anything

* I suppose it worked under Windows 7 earlier, but it might have been Release Candidate of Windows 7, now I'm using the real thing.