use IPC::Cmd qw[can_run run run_forked];
my $cmd = "systemctl --failed --no-legend";
                                          
my ( $success, $error_message, $full_buf, $stdout_buf, $stderr_buf ) =
  run( command => $cmd, verbose => 0 );
                                              
if ($success) {
    my $nr_els = @$full_buf;
    if ( $nr_els == 0 ) {
        print "OK: all systemd units in their desired state\n";
    }
    else {
        print "WARNING: ";
        print "$nr_els systemd units not running\n"; 
        for (@$full_buf) {
            print "$_\n";
        }
    } 
}


##</code><code>##

WARNING: 1 systemd units not running
avahi-daemon.service loaded failed failed Avahi mDNS/DNS-SD Stack
packagekit.service   loaded failed failed PackageKit Daemon