Yes, there is, I have found it!

This code spawns a new process with its STDIN and STDOUT redirected to regular pipes/sockets and also attaches a PTY to the child to simulate manual interaction that is handled by Expect.

my $pty = IO::Pty->new;
my $expect = Expect->init($pty);
my $child = open2($in, $out, '-');
if (defined $child and !$child) {
    $pty->make_slave_controlling_terminal;
    exec @cmd;
    exit -1;
}
unless (defined $child) {
    die "unable to spawn new process";
}
$expect->expect($timeout, "...");
...

# and once the authentication phase is over, $in and $out are used for
+ the rest of the IPC.
[download]

Also, I have to retain the PTY open until the child program finishes (not just after the authentication is over), otherwise, it gets a SIGPIPE and exits.

Passing a dash (-) to open2 (from IPC::Open2 or open3 from IPC::Open3), makes it fork the current Perl process instead of running an external program.