I'm not sure whether this is relevant but I had a problem recently when adding some housekeeping code to a script. The script continuously monitored something, writing stuff to a log file. I decided to fork a child process to do housekeeping on old logs and I set $SIG{ CHLD } = q{IGNORE} in the parent before forking as I did not want the parent to interrupt its monitoring while waiting for the child to finish.

Inside the child I was using system to invoke compression on selected log files and deleting others, logging what it was doing to a separate file. I could see from the log that system was successfully invoking bzip2 and the file was being replaced on disk by a compressed version but, strangely, the script was detecting a non-zero exit status and was logging a failure. Further investigation showed that $? contained -1 but that didn't seem to fit with the fact that the child (bzip2) did successfully launch. It turned out that the problem was caused by the $SIG{ CHLD } = q{IGNORE} which caused system to return a -1 exit code. $! was being set and, IIRC, the error was "No child process" or something like but I can't be sure of that. I think the -1 exit code was being returned because, since children were ignored, exit codes were of no use. The solution in my script was to do local $SIG{ CHLD } = q{DEFAULT} in each scope where I was going to do a system call.

None of this explains why you should suddenly get strange behaviour on just one server or why only qx{ ... } is affected. I should explain that I tried substituting qx{ ... } for system( ... ) without success before I got to the root cause of my problem. Although it doesn't seem that your problem is closely connected to the one I had, I thought it had enough parallels to make it worth mentioning here in case it gave you any clues. The platform was RHEL 5.4 with perl 5.8.8 interpreter.

I hope this is helpful.