This is a good point. If you don't explicitly close(STDERR), it should stay at file descriptor 2 when you re-open it. For example:

close(STDIN);
close(STDERR);
open(STDERR, ">/tmp/foo");
print fileno(STDERR), "\n";  # prints 0
[download]

However, if the close(STDERR) call is removed, STDERR remains at file descriptor 2.

A quick test of Proc::Daemon indicates that STDIN, STDOUT and STDERR all have the expected file descriptors.