Preview != create :(

Capture::Tiny is indeed the way to go, with a small gotcha:

$ cat test.pl
use 5.18.2;
use warnings;

use Capture::Tiny "capture";

my ($out, $err, $ext) = capture {
    # Local scope
    print STDERR "Output to STDERR\n";
    warn "Warn!\n";
    system "ls /tmp/bjfgerfbv";
    qx{ls /tmp/jhgbejgrbe};
    };

# Previous scope
print STDERR "Hello world\n";
system "ls /tmp/foobar2";

print STDERR "OUT: $_\n" for split m/\n/ => $out;
print STDERR "ERR: $_\n" for split m/\n/ => $err;
$ perl test.pl
Hello world
/bin/ls: cannot access /tmp/foobar2: No such file or directory
ERR: Output to STDERR
ERR: Warn!
ERR: /bin/ls: cannot access /tmp/bjfgerfbv: No such file or directory
ERR: /bin/ls: cannot access /tmp/jhgbejgrbe: No such file or directory
[download]

The gotcha is that STDERR inside capture is not unbuffered, so if your last print STDERR "foo"; does not have a trailing newline, it is lost. That might change in a newer release (at least I hope)

The child process cannot access variables or call subroutines in your process. So your second criteria cannot be met. You will need to have the child write to a pipe from which you will read and write to the tied handle.

use IPC::Open3 qw( open3 );

open(local *CHILD_STDIN, '<', '/dev/null')
   or die $!;

my $pid = open3(
   '<&CHILD_STDIN',
   '>&STDOUT',
   local *CHILD_STDERR,
   'ls file-that-does-not-exist.txt',
);

while (<CHILD_STDERR>) {
   print $tied_fh $_;
}

waitpid($pid, 0);
[download]

OK, except here's where I'm unclear. This command sorta does what I want:

open (STDERR, '>', './log.txt')
   or die "could not open STDERR: $!\n";

It captures STDERR from child processes without any calls to system or backticks or whatever having to be called in a special way. The problem is that it only redirects to a file. Isn't there some way to redirect it to a subroutine or file handle instead?

It captures STDERR from child processes without any calls to system or backticks or whatever having to be called in a special way.

That's because child processes inherit their stdin/stdout/stderr from their parent processes.

But file handles are OS level entities, not language specific entities. A tied filehandle is a perl-level entity; it cannot be inherited by a C program.

---

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority". I'm with torvalds on this

In the absence of evidence, opinion is indistinguishable from prejudice. Agile (and TDD) debunked

This sounds promising.

From perl's core documentation: open

Here is a script that saves, redirects, and restores STDOUT and STDERR
+ using various methods:
    #!/usr/bin/perl
    open(my $oldout, ">&STDOUT")     or die "Can't dup STDOUT: $!";
    open(OLDERR,     ">&", \*STDERR) or die "Can't dup STDERR: $!";
    
    open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
    open(STDERR, ">&STDOUT")     or die "Can't dup STDOUT: $!";
    
    select STDERR; $| = 1;  # make unbuffered
    select STDOUT; $| = 1;  # make unbuffered
    
    print STDOUT "stdout 1\n";  # this works for
    print STDERR "stderr 1\n";  # subprocesses too
    
    open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
    open(STDERR, ">&OLDERR")    or die "Can't dup OLDERR: $!";
    
    print STDOUT "stdout 2\n";
    print STDERR "stderr 2\n";
[download]

Bill

Isn't there some way to redirect it to a subroutine

The child process cannot access variables or call subroutines in your process. You will need to have the child write to a pipe from which you will read, then you can pass what you read to a subroutine.

or file handle instead?

huh? "Redirecting data sent to a file handle to a file handle" makes no sense.

The Capture::Tiny docs indicate that it should work at least with some tied filehandles, maybe you could use that, or at least look into its source code to help you figure out to do what you want?