There are a number of ways that the number of open files is limited.

The perl process may have hit a process limit for open files. The web server may have hit a process limit for open files if you're running inside the web server's process. The user that runs the perl system may have hit a per-user open files limit. Some operating systems have a limit on the number of open files system wide, but I'm not sure any modern ones do.

The command ulimit -a will tell someone all about the soft user limits set on processes spawned under a particular shell, which is usually a user's login shell, under Bash or ksh on a Unixy OS. ulimit -a -H will show the hard limits. ulimit -n shows specifically the number of open file handles allowed per process and can be used with -H as well to see the hard limit for that.