cmd-line file frobbing w/ perl   (link)

http://www.perl.com/pub/a/2004/10/14/file_editing.html
If you only want to read in one or more files, apply a regex to the contents, and spit out the altered text as one big stream...

If you want to edit the files in place, editing each file before going on to the next, that's pretty easy, too...