Tip: Use a canned CPAN package like File::Find to find all the files you want to rename, unless there is a truly outrageous number of them, and put the file names into a list or array before going back through that list to move or rename the files. On some operating systems manipulating files in the directory that you are scanning can cause files to be found more than once and/or for other files to be skipped.
thx A.M., these files aren't hard for me to find because they're ones that I will have caused to have been recently-downloaded. I almost want to use it as my soapbox for "The Day in Internet Images."