in reply to
How to handle a browser window that opens up during the execution of some external program commands
Post the Powershell code that launches and manipulates IE, and I'll tell you how to handle the trivial part of filling in fields. The PS code is already using something to automate IE, and... oh wait, this is a Perl forum, not a Powershell forum.
If you must take control of a window using Perl, on Windows, that was launched by another process (Powershell in this case), then Win32-GuiTest is probably your best bet...