Now that i look at it, i see a couple of great things :

- the structure of %devs and the way it's built from %comp

- the way you "each" through %dev : $keys is a hash ref, (with the adresses of a device as keys). I need to 'each' more, when i have refs as values !

So, now, the grep. It's over the keys of refcomp, so $_ is an adress. Hence, if $keys->{$_} is not defined, it means that this device we are eaching on is missing for that adress. keys %comp has all adresses but keys %$keys only has the adress of a specific device.

Honestly, i would never have found that myself, the way it works for any number of adresses by 'reversing' the hash and looking at it from the device ... it's brilliant :D
It's also a very consistent way of looking at this specific system conf giving me a complete 'for sure' list of errors. Thanks !!