Hmmm... I've never noticed fragmentation being a source of unreasonable latency in an HTTP environment on a *n(i|u)x machine, but I'll take your word for it. Moving right along:

I would never think to put images into my CGI directory -- they'd live out in the public HTML directory tree somewhere, and if they were coming from untrusted sources that directory would be in a partition mounted with the noexec option. The CGI application, if it has to, should be able to reach into that directory just fine, it being under HTTP document root. I'd still think very seriously about $q -> redirect($image_url); though.

Back to the original problem: Without more information it's impossible to be certain, but if it were mine to fix I'd start by looking at whether or not there's a source of Unicode data in the system, based upon that log line you got when you stuffed a warn into the code.