STDIN, STDOUT, and STDERR are shared

Both the main process and the backgrounded one (the "child" process) share the same STDIN, STDOUT and STDERR filehandles. If both try to access them at once, strange things can happen. You may want to close or reopen these for the child. You can get around this with opening a pipe (see perlfunc/"open") but on some systems this means that the child process cannot outlive the parent.

Signals

You'll have to catch the SIGCHLD signal, and possibly SIGPIPE too. SIGCHLD is sent when the backgrounded process finishes. SIGPIPE is sent when you write to a filehandle whose child process has closed (an untrapped SIGPIPE can cause your program to silently die). This is not an issue with system("cmd&").

Zombies

You have to be prepared to "reap" the child process when it finishes

    $SIG{CHLD} = sub { wait };

See perlipc/"Signals" for other examples of code to do this. Zombies are not an issue with system("prog &").

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