#!/usr/bin/perl
use strict;
use warnings;
use IPC::Open3;
use IO::Select;

my @cmd = qw'echo bar';  # writing to 'echo' fails
#my @cmd = qw'cat';      # ... while 'cat' works

open3(my $wh, my $rh, undef, @cmd);

# tiny delay, so the command gets enough time to close STDIN
# (prevent potential race condition, in this demo script)
select undef,undef,undef, 0.1;

my $msg = "foo";
#-----
print $wh "$msg\n";
close $wh;
#-----

my $data = "";
if (IO::Select->new($rh)->can_read()) {
    print "reading from $rh ('@cmd |')\n";
    while (sysread($rh, my $buffer, 1000)) {
       $data .= $buffer;
    }
}
print "data: $data\n";