# this would try to re-use STDERR's file descriptor 2,
# which dies with a "Bad file descriptor" error
open STDERR, ">", \$variable or die $!;

# closing STDERR first 'detaches' STDERR from file descriptor 2
# so it is then being re-opened to an internal file handle
printf "fd=%d\n", fileno(STDERR);   # 2
close STDERR;
open STDERR, ">", \$variable or die $!;
printf "fd=%d\n", fileno(STDERR);   # -1  (not accessible from system()'s child process!)