You might try IO::Pty to fake it out. Here is a simple example, but you might want to google for better ones. 
#!/usr/bin/perl
use IO::Handle;
use IO::Pty;

sub do_cmd() {
    my $pty = new IO::Pty;
    defined( my $child = fork ) or die "Can't fork: $!";
     if ($child){
          $pty->close_slave(); #needed to close
          return $pty;
    }
    

    POSIX::setsid();
    my $slave = $pty->slave;
    close($pty);
    STDOUT->fdopen( $slave, '>' ) || die $!;
    STDERR->fdopen( \*STDOUT, '>' ) || die $!;
    system("echo This is stdout output from an external program");
    exit 0;
}

my $fh = do_cmd();
while (<$fh>) { print; }

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In reply to Re^3: open3 buffering in linux vs. os x by zentara
in thread open3 buffering in linux vs. os x by Lexicon

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