bobafifi has asked for the wisdom of the Perl Monks concerning the following question:

A friend of mine made her original website using FrontPage Express 2.0 circa 2000 before moving to Blogger. Today, she's got some 300 URLs from that old site that are returning "404 Not Found". If possible, I'd like to batch remove them (there are about 100 .htm pages) and replace with a simple 404 Not Found notice. I know how to remove individual URLs from the pages using a find/replace one liner, but doing them all in one pass has so far eluded me. Any advice about where to look for more info about how best to go about this would be greatly appreciated. Thanks!

UPDATE 10/27/17
Thanks everybody for all the help and good ideas! Turns out the problems I was having with running the one-liner were with malformed URLs and had nothing to do with the code. Once that was fixed, I just stacked the 300 one-liners on top of each other and it ran perfectly. Like so:
find . -type f -name "*.htm" -print|xargs perl -i -pe 's/http:\/\/exam[404 Not Found]/g'; find . -type f -name "*.htm" -print|xargs perl -i -pe 's/http:\/\/exam[404 Not Found]/g'; find . -type f -name "*.htm" -print|xargs perl -i -pe 's/http:\/\/exam[404 Not Found]/g'; etc.