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>
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.