HTTP-404 appears to have multiple new-lines in $post. But your use of $ in your regex will only find the last one. To make the $ work for mid-string new-lines, you would need to use the /m modifier. And since you are attempting to use \n (and perhaps \r) for your match, the $ is superfluous to your strategy anyway.

Also, Perl automagically hides the difference between OS's handling of new-lines. I can match \n on my windows machine with no problems. Perl pretends it's a single character and hides the ugly details from me. To put it another way; \n is an OS-compliant newline, not linefeed. So you don't need to worry about \r. Trying to match it will confuse things.

Finally, if we wanted to depend on $ for our solution, we might go for: $post =~ s/$/<br>/mg; which is sweeter than the regex in my original post.