Definitely server side, but hey, got some more information to help you debug: 

$ GET -S http://tis.spaghetticoder.org/test/e34/M5_%28S38%29_SAL/Brake
+s/Brake_lines/
GET http://tis.spaghetticoder.org/test/e34/M5_%28S38%29_SAL/Brakes/Bra
+ke_lines/ --> 200 OK
<html>
<head><title>handler</title></head>
<body>
Hello from ME.<br />
</body>
</html>
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html><head>
<title>200 OK</title>
</head><body>
<h1>OK</h1>
<p>The requested URL /s/friendlyNew.pl was not found on this server.</
+p>
</body></html>

[download]

Note that the server returned a 200 status with two responses merged into one.

In reply to Re^3: Different context by ikegami
in thread mod_perl give 404 not_found with content by Jaap

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