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 ...

As soon as you receive payment to your PayPal account, PayPal posts the transaction details to your specified URL, which you either configure in your PayPal preferences, or in your HTML forms' "notify_url" hidden field........ Business::PayPal::IPN is the library which encapsulates all the above complexity into this compact form:

So it looks like it is too simple

so where is it getting the params that are in the incoming POST from?

It creates its own CGI internally.