You need to make sure fileno(STDERR) is equal to 2.

fileno()==0 will be used as the child's STDIN.
fileno()==1 will be used as the child's STDOUT.
fileno()==2 will be used as the child's STDERR.

The above will allow you to redirect STDERR to a file, but system file handles (as opposed to Perl file handles) can't be redirected to a Perl function (such as the log4perl handler). If you wish to do that, you'll have to use a temporary file, IPC::Open3 or something like pc88mxer's code.

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
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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.

As for STDOUT and STDERR when using Proc::Daemon, it just redirects them to /dev/null as a convenience. There's no problem re-opening them to other files. Closing those handles first is not necessary. When attempting to open an already open file handle, perl will close it first.

If the open STDERR ... call is not working in daemon mode, perhaps it's because it can't write to or create "$run_path/stderr.txt". What are the permissions on the directory and file, and what userid is the script running as in daemon mode?

Great, I misunderstood your code at first, after looking through the manual and your explanation it now makes sense. I'm going to implement it like this and I'll report back the status. Thanks for your time & help.

#!/usr/bin/perl

use Proc::Daemon          qw( );
use File::Basename        qw( dirname );
use File::Spec::Functions qw( rel2abs );

my $homedir;
BEGIN { $homedir = dirname(rel2abs($0)); }

{
   Proc::Daemon::Init;
   chdir($homedir);
   open STDERR, '>', 'stderr.txt';
   system(q{perl -e'print STDERR "mwuhahahaha\n"'});
}
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$ 688767.pl
$ cat stderr.txt
mwuhahahaha
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Actually, this works perfectly. For some reason it wasn't working yesterday but it is working today with no code changes. I must have been running an older version or something.

This is great to just redirect stderr & stdout to individual logs. I'm going to explore the first answer to my question on getting this working with log4perl.

Thanks for your time & replies.