this is more academic question, since I've found that an IPC module does what I needed.

basically i wanted to execute a system command and get its stderr, stdout, and return value. so i invoke the command with backticks in two different subroutines, one of which redirects STDERR to a variable, and the other doesnt.

when i dont touch stderr, the invoked command prints the command's stderr to the script stderr. but when i try to capture the script stderr, the command's stderr disappears altogether. this baffles me, because if the script itself sends to stderr (ie: the warn builtin), it gets captured just fine.

#!/usr/bin/perl -w

use strict;
use Data::Dumper;

# execute command without capturing STDERR
sub exec_nocap {
    # execute command via backticks, getting output (STDOUT) and retur
+n value
    my $command = shift;
    my $out = `$command`;
    my $return = $?;
    
    # dump everything (in an anonymous hash)
    print Dumper {
        'Return value' => $return,
        'STDOUT' => $out,
    };
}

# execute command, capturing STDERR
sub exec_cap {
    # dupe STDERR, to be restored later
    open(my $stderr, ">&", \*STDERR) or do { print "Can't dupe STDERR:
+ $!\n"; exit; };
    
    # close first (according to the perldocs)
    close(STDERR) or die "Can't close STDERR: $!\n";
    
    # redirect STDERR to in-memory scalar
    my $err;
    open(STDERR, '>', \$err) or do { print "Can't redirect STDERR: $!\
+n"; exit; };
    
    # just to demonstrate that STDERR capturing is working
    warn('this is a warning');
    
    # execute command via backticks, getting output (STDOUT) and retur
+n value
    my $command = shift;
    my $out = `$command`;
    my $return = $?;
  
  # restore original STDERR
  open(STDERR, ">&", $stderr) or do { print "Can't restore STDERR: $!\
+n"; exit; };
  
  print Dumper({
    'Return value' => $return,
    'STDOUT' => $out,
    'Redirected STDERR' => $err,
  });
}

exec_nocap('perl -e "die(\'this is a fatal error\')"');

print "\n"x5;

exec_cap('perl -e "die(\'this is a fatal error\')"');
[download]

as i said, this is purely an academic question, but why is this happening?

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