#!/usr/bin/perl

use strict;
use IO::Handle;

autoflush STDOUT 1;  # had to do this, even tho Open3 doc said it would be done already

while (<>) {
    chomp;
    print "line $. to STDOUT: $_\n";
    warn "mesg $. to STDERR\n";
}

##</code><code>##

#!/usr/bin/perl

use strict;
use IPC::Open3;

my $cmd = "test-child.pl";
my ( $in, $out, $err );

$err = "true";   # any string will do (don't use a number)

my $pid = open3( $in, $out, $err, $cmd );

my ( $outlog, $errlog );
for ( 1..3 ) {
    print $in "data $_\n";
    $outlog .= <$out>;
    $errlog .= <$err>;
}
close $in;
waitpid $pid, 0;

print "outlog contained:\n$outlog\n";
print "errlog contained:\n$errlog\n";

__OUTPUT__
outlog contained:
line 1 to STDOUT: data 1
line 2 to STDOUT: data 2
line 3 to STDOUT: data 3

errlog contained:
mesg 1 to STDERR
mesg 2 to STDERR
mesg 3 to STDERR