#!/usr/local/bin/perl -w

use strict;
use Net::FTP;

{
   open (FH, ">debug") || die $!;
   *STDERR = *FH; 

   my $ftp = Net::FTP->new ("linux", Debug => 1) || die $!;

   $ftp->login ("foo", "bar");
   $ftp->quit;

   close (FH);
}
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BEGIN {
  open(STDERR, ">stderr.log");
}
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That probably wanted to be :-

BEGIN {
  open(STDERR, ">>stderr.log")
    or die "Failed to open log file";
}
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Note the >> and the or die.

You could do this with a tied file handle too, as demonstrated recently by tilly - here

#!/usr/bin/perl -w

use strict;
use POSIX ":sys_wait_h";

BEGIN {
    open(STDERR, ">>stderr.log") or die "Bad, can't open log: $!";
}

die "Can't fork!" unless (defined(my $pid = fork()));
if ($pid) {
    my $kid;
    do {
        $kid = waitpid(-1, &WNOHANG);
    } until $kid == -1;
} else {
    exec "./realprog.pl";
}
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# Takes a filehandle and filename and has output to that handle go
# to STDERR and to that filename.  It will work even if the handle
# being tied is STDERR itself.  No escaping is done for the filename,
# caveat programmer
sub tee_to_stderr {
  my $file = shift || die "No filename passed\n";
  my $ip = open (STDERR, "| perl") || die "Cannot pipe to Perl: $!";
  my $fh = select(STDERR);
  $| = 1;
  select($fh);
  print STDERR qq(
    unless(open (OUT, ">$file")) {
      print STDERR "Cannot write to $file: \$!\n";
      kill 9, getppid();
    }
    select(STDERR);
    \$| = 1;
    select(OUT);
    \$| = 1;
    while (<>) {
      print OUT;
      print STDERR;
    }
  );
  print STDERR "\n__END__\n";
}
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The idea is to not just redirect STDERR, but also to have it still going to STDERR so that you get your log and they still see their own errors.

The right way to do this should use IO::Tee, but the problem is that one of the filehandles you want to tee is STDERR. This gets us into the same recursion that you saw in Filehandle Filter. I don't have the energy at the moment to offer a good solution, though I am sure there is one and that I will be very irritated with myself when I see it...

UPDATE
I am indeed irritated. Fastolfe pointed out that you can always dup a filehandle:

open(NEW, ">&OLD") or die "Cannot dup OLD: $!";
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Sometimes it's easier to let the shell work for you. Have you considered writing a shell script that neatly wraps up the 2>stderr.log?

ObFastolfeDisclaimer: You may want to use 2>>stderr.log instead.

Thats a bit difficult in this case because I would have to propagate command-line arguments from the wrapper to the real script.... ACtually maybe not:

@syscmd = ('real-prog', @ARGV);
system @syscmd or die "syscmd failed";
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