You need to break this into two parts -
1.the Win32 app sending an event to another app.
2.the Perl::Tk app receiving the event.

For part 2, you need to use

Tk::Receive($widget, "command");

That bit I got from O'Reilly Learning Perl/Tk, which recommend using -T to check for taints, in case the command is arbitrary code - that is, your tk app gets hacked by someone's Win32 or Tk app. It also recommends you do careful checking of command - e.g. don't just do eval("command") without checking for commands that contain things like

system("rm -rf");

For Perl::Tk talking to other (Tk?) apps use send

$widget->send("application" => callback);

Use -async to not wait for callback to execute.

For part 1, I have no idea.