though I've got to do some man page reading to fully understand this. I've never used any of the thread modules

A little explanation might help.

The core of the mechanism is here:

## Start the command connecting our output to its input
## (You might need to use open2/3() if you want to capture the output)
my $pid = open CMD, '|-', $cmd or die $!;

## A shared var that captures whether the print succeeds
## If it does, the process went into an input state
## if it doesn't it ended (or was terminated) without entering an inpu
+t state
my $inInputState :shared = 0;

## Attempt to write to the process
## in a thread so we can do other things while it blocks.
## No newline so the process doesn't see it
## a series of spaces followed by backspaces 
## which should be "cancelled out" by the line edit API
## 4096 chars to ensure it gets through pipe buffering
async {
    $inInputState = 1 if printf CMD " \b"x2048;
}->detach;

## A microsleep to ensure responsiveness whilst avoiding cpu burn
## until the process self terminates
## or we reach the timeout period
## or the print succeeds -- entered input state
Win32::Sleep 10 until !kill 0, $pid
    or $timedOut = time() > $timeout
    or $inInputState
[download]

The rest is just mechanics.