Apologies in advance if I am misunderstanding you, but are you having difficulty with holding a key in and then doing some other kind of SendKeys/SendMouse action? I have provided two examples here, the second I don't really have time to take any further, I will leave that as an exercise for you.

#!/usr/bin/perl

use Win32::GuiTest qw(:ALL :VK);    

# launch notepad, hold shift, type things
system("start notepad.exe");
sleep 1;

SendRawKey(VK_SHIFT, 0);
SendKeys ("im the operator with my pocket calculator");
SendRawKey(VK_SHIFT, KEYEVENTF_KEYUP);

# Untested, but you may be able to get away with something like this
SendRawKey(VK_ALT, 0);
SendLButtonDown();
SendLButtonUp();
SendLButtonDown();
SendLButtonUp();
SendRawKey(VK_ALT, KEYEVENTF_KEYUP);
[download]

Hope this helps

Martin