Maybe this will get you started. Save it as 700695.pl and run it from the directory where you saved it:

#! perl -slw
use strict;
use Win32::GuiTest qw[SendKeys FindWindowLike SetForegroundWindow];

## Start asynchronously
system 1, 'notepad';
sleep 1; ## Give it chance to start

## Find it's window id
## Assumes only one copy with 'Untitled' is running.
## It two copies are, it doesn't matter which to use anyway
my $wid = ( FindWindowLike 0, "Untitled - Notepad", 0, 0 )[0];

## Ensure it's on top
SetForegroundWindow( $wid );

sleep 1; ## Give it chance to respond

## Send the commands needed.
## If the processing is slow it may be better to break them into chaun
+ks
## and intersperse with sleeps
SendKeys
    ## Open the file menu, then the open dialog, 
    ## enter the name of the file, and hit alt-O
    "%FO 700695.pl %O"

    ## Move to the end, add a DATA section and some text
    . "^{END}__DATA__~This was added automatically using Win32::GuiTes
+t~"

    ## Open the file menu and save. OPen the file menu and exit.
    ## Say Yes to quiting
    . "%fs %fx Y"

    ## A short delay between keystrokes can help with slow dialogs.
    , 50;
[download]

When you run it, it should get the following appended to it:

__DATA__
This was added automatically using Win32::GuiTest
[download]