$ ls -l
total 0
-rw-r--r-- 1 john john 0 Feb  1 20:28 four
-rw-r--r-- 1 john john 0 Feb  1 20:28 one
-rw-r--r-- 1 john john 0 Feb  1 20:28 two ? three
$ perl -e '$counter = 0; for ( <*> ) { $old = $_; $new = "caption" . "$counter" . ".txt"; rename( $old, $new ); $counter++; }'
$ ls -l
total 0
-rw-r--r-- 1 john john 0 Feb  1 20:28 caption0.txt
-rw-r--r-- 1 john john 0 Feb  1 20:28 caption1.txt
-rw-r--r-- 1 john john 0 Feb  1 20:28 caption2.txt