What's the proper way to safely run this error-prone system call in perl?

Answer 1

If a command might go haywire and hang your script, a good defense is to use alarm and a SIGALRM handler to make the command time out.

sub system_with_timeout {
    my ($timeout, @command) = @_;
    local $SIG{ALRM} = sub { die "Timeout\n" };
    my $result;
    if ($timeout > 0) { alarm $timeout; }
    eval {
        $result = system(@command);
    };
    if ($@ =~ /Timeout/) {
        warn "@command timed out.\n";
        return -1;
    }
    return $result;
}

Check the return values to make sure your command was successful or whether it timed out.

my $z = system_with_timeout(30, "systemsetup -setusingnetworktime off");
if ($z == 0) {
    # success ...
} elsif ($z == -1) {
    # timed out ... try it again ?
} else {
    # failed for some other reason ...
}

Answer 2

If you don't care about the error itself (e.g. it is just a warning message), by all means re-direct the STDOUT/STDERR of the command; although I'd personally recommend re-directing to a /tmp logfile (and not /dev/null) in case you ever need to troubleshoot some OTHER error.

If you actually wish to take a specific action upon such an error, you can either check the return code (see mobrule's answer), or if the command always returns 0, capture the putput and filter it for error:

my @script_out = `systemsetup -setusingnetworktime off`;
# better yet use open with a pipe instead of backticks 
if (grep {/writeconfig.*dropping incoming DO message/} @script_out {
    # do error handling
}