In 5.32: How do I close a file descriptor by number?, I believe that the returned status is incorrectly handled.

# die "can't sysclose $fd: $!" unless $rc == -1;
# should be
die "can't sysclose $fd: $!" if $rc == -1;
# or
die "can't sysclose $fd: $!" if $rc;
[download]

From man 2 close:

RETURN VALUE
       close returns zero on success, or -1 if an error occurred.