Because when you open a in handler it opens file and assigns to it descriptor with first available number -- 2 (STDERR).

Update: try this:

#!/usr/bin/perl 
use strict;
use warnings;

close STDERR;
open FH, '>', 'log';
die "exit from program\n";
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Update2: or more correct method:

#!/usr/bin/perl 
use strict;
use warnings;
use POSIX qw(dup2);

open FH, '>', 'log';
dup2(fileno FH, 2);
die "exit from program\n";
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But actually I prefer using Log::Log4perl and its logdie method.

Update3: uhm, I just discovered, that when you open STDERR perl always assigns it to descriptor 2:

#!/usr/bin/perl 
use strict;
use warnings;
use POSIX qw(dup2);

open FH, '>', 'log';
dup2(fileno FH, 2);
open STDERR, '>', 'stderr';
# this would be placed into 'stderr', not into 'log'
warn "STDERR descriptor is:", fileno(STDERR), "\n";
die "exit from program\n";
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Good grief. This seems to imply that if you close STDERR then die (and other diagnostics ?) will happily be written to whatever happens to be attached as descriptor 2 ?

If so, then the trick appears to be to open STDERR '>', '/dev/nul', or something else to occupy descriptor 2 to squelch output ? Thinking about a more portable solution I tried:

  use strict ; use warnings ;

  sub die_handler {
    open FH, ">&1";
    print FH "die_handler: ", @_ ;
  } ;

  $SIG{__DIE__} = \&die_handler ;

  close STDERR ;
  open STDERR, '<', \'' ;

  die "Oh shoot !" ;
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which appears to do the trick. Noting that the close STDERR is required.

You could close FH in the handler, in which case the subsequent output to stderr (produced by the die statement the usual way) won't end up anywhere, neither in the file 'a', nor on stderr, because file descriptor 2 (which is used for both FH and STDERR in this case — as zwon pointed out) would then no longer be open at that point in time.  Or, as oshalla suggested, simply reopen STDERR to /dev/null.

I just tried the code and, as one would expect, got the same as lakslunananindia. But if I close FD within the sub handler(), after wrting to it, then it looks like the problem is also solved without fiddling with STDERR so much.

Is there something wrong with closing FD in the signal handler?

I don't have any real experience with signal handlers (though by now I should have had some experience); but coincidentally, I just began exploring them a couple of months ago. I don't get many opportunities to do much exploration, though. So this little node is most instructive for me.

Thanks, all.

perl -e 'BEGIN{close(STDERR)};open FH,">log"; die 'testing';'
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