Is it enough to merge the stdout together with the stderr into one stream? If so then you could do

$output = `$cmd 2>&1`

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It sounds like you want to capture the stdout and stderr into seperate variables though. You will need this: 
use strict;
use IO::Select;
use POSIX;

pipe(STDOUT_READ, STDOUT_WRITE) || die;
pipe(STDERR_READ, STDERR_WRITE) || die;

my $pid = fork();
die unless (defined($pid));
if ($pid == 0) {
    close STDOUT_READ;
    close STDERR_READ;
    open(STDOUT, ">&STDOUT_WRITE");
    open(STDERR, ">&STDERR_WRITE");
    close STDOUT_WRITE;
    close STDERR_WRITE;
    exec $cmd;
    POSIX::_exit(1);
}

my $stdout_collected = "";
my $stderr_collected = "";

close STDOUT_WRITE;
close STDERR_WRITE;

my $sel = IO::Select->new();
$sel->add(\*STDOUT_READ);
$sel->add(\*STDERR_READ);

my $remaining = 2;
while ($remaining > 0) {
    my @ready = $sel->can_read();
    for my $r (@ready) {
        my $got;
        if (sysread($r, $got, 1024) == 0) {
                close $r;
                $remaining--;
        }
        ($r == \*STDOUT_READ) ?
                $stdout_collected :
                $stderr_collected .= $got;
    }
}

undef $sel;
waitpid $pid, 0;

# stdout is collected in $stdout_collected
# stderr is collected in $stderr_collected

[download]

Update: Changed read to sysread

In reply to Re: Capturing stderr by Celada
in thread Capturing stderr by Zadeh

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