I'm not sure how one would diagnose this behavior without seeing some of the code, or (preferably) some snippet that demonstrates an equivalent behavior.

Is your script running on a server where you can look to see what perl version is pointed to by the path on the shebang line? Do you have control over (or up-to-date info about) the perl installation and modules on the server? If not, try including something like print $] in your script, to see the perl version somewhere on the page that it creates; maybe someone was trying a different perl version for a day or so, then reverted or installed a patch (or did it right the second time)...