Ah, ok, now I kinda get what you want. But what about a simple something like this?

print header;
print <<EOF
<script language="JavaScript">
alert('Hello World');  
history.go(-1);
</script>
EOF
;
[download]

This way, you will get your popup and the user will go straight back to the previous page which probably is still cached (if JavaScript is turned on ... of course). For simple forms, this should be ok.