As a "one-liner" (with caveats), this would do the trick:

perl -wle "$x='000'; $x++ and $o=$_ and s[/(.*$)][/$x-$1] and rename($
+o,$_) or warn $! for glob qq[$ARGV[0]/*]" dir
[download]

but trying to remember how to type that is too hard, so you'd want to make it a script, at which point your original is much nicer.