My two trains of thought were:

1. Your shell's idea of where stdout was directed had changed because you were inheriting a shell alias, or environment variable that was explicitly redirecting stdout when you invoke your script.

   That print STDOUT 'yada'; appears in the right place when the bug is not manifesting itself probably precludes this idea.

   Though, the perl debugger has a remote debugging option that might cause strange phenomena if it was being invoked silently?

2. The other was that some module, like CGI::Carp that redirects STDERR to some other place (a log file or fatalstobrowser (I didn't receive any:)) and does it during the BEGIN{} phase, might be a possibility.

Beyond that, the best I could advise, is to recreate the error that doesn't produce a message, and then slowly strip as much out of the script as possible whilst retaining the errant behaviour.

Once you have something of a postable size, without to many non-standard dependancies, you could post that here and we would have an opportunity to try it and see if the problem is something confined to your system, version of Perl etc. Or a more generic problem.