print can result in system calls. $! is meaningless after the first print.

$ perl -wle'
$SIG{CHLD}="IGNORE";
$! = 0;
my $rv = system("ls -d .");
my $errno = $!;
print $rv;
print 0+$errno;
print $errno;
print 0+$errno;
'
.
-1
10
No child processes
10
[download]

Note that 0+ is much easier to understand than -+-, and it conveys the intent much better.