sub make_a_worker
{
    my ( $postback, $message ) = @_;
    POE::Session->create(
        inline_states => {
            _start              => \&worker_start,
            worker_child_stdout => \&worker_child_stdout,
            worker_closed       => \&worker_closed,
            sig_child           => \&sig_child,
        },
        args => [ @_ ],
    );
}

sub worker_start
{
    my ($kernel, $session, $heap, @args ) = @_[KERNEL, SESSION, HEAP, ARG0 .. $#_];
    $heap->{postback}=$args[0];
    $heap->{message}=$args[1];
    $heap->{MessageID} = $args[1]->getHeaderField('MessageID');
    $heap->{SessionID} = $session->ID;
# ... your code as is...
    print "  Message ID: ".$heap->{MessageID};
    print "  -start_message_process (end): ".$heap->{SessionID}."\n";
}
# ....
sub worker_closed
{
    my ( $kernel, $heap) = @_[ KERNEL, HEAP];
    my $postback=$heap->{postback};
    my @results=@{$heap->{results}};
    # I don't know how $heap->{results} is created, but I presume it is based somehow on the output of your process
    
    $postback->(@results);

# ... anything you need (yield('sig_child') is not one of that)   
}