perl – 为什么system()返回0,即使它执行的程序死掉

#!/usr/bin/perl
# test_lock
use strict;
use warnings;

( my $code = <<'CODE') =~ s/^s+//gm;
    #!/usr/bin/perl
    use strict;
    use warnings;
    use Fcntl qw(:flock);

    # Make sure only one instance of the program is running at a time.
    open our $Lock,'<',$0 or die "Can't lock myself $0: $!";
    flock $Lock,LOCK_EX | LOCK_NB
      or die "Another instance of $0 is already running. Exiting ...n";

    sleep(2);
CODE

my $progfile = '/tmp/x';
open my $fh,'>',$progfile or die $!;
print $fh $code;
close $fh;

$|++;
my $ex1 = system("perl $progfile &");
print "First system(): $ex1n";
my $ex2 = system("perl $progfile");
print "Second system(): $ex2n";

我期望第二次调用system()会返回一个非零值($ex2),因为它无法获得锁定并死掉.但是我得到：

$perl test_lock
First system(): 0
Another instance of /tmp/x is already running. Exiting ...
Second system(): 0

我的假设有什么问题？ (有没有更好的方法来测试$代码？)