I think that I do know what it is. I've been reading the manuals for a while, and have implemented it before. Hence I don't see how the example given in the synopsis can possibly work, unless it somehow incorporates a web server to receive the incoming IPN POST req from PP (in which case where is the config for that?)

I would have expected something like:

my $hash = $cgi->params(); # the incoming POST params
my $ipn = Business::PayPal::IPN->new( $hash );
if ( $ipn->isOK ) {
 # further checks here ...
} else {
 # invalid, log it
}

Basically, it needs to know something about your config and the post to work, where is that info?

Or maybe PayPal really does work like magic ...