i'm not 100% shure, but i'll try to explain afaik.
$_->{$.}
this only makes $_ a reference to a hash. when accessing $_ via scalar context (as the folloing regex does) you get something like "HASH(0x814cc20)".
s:.?(.)..(.):print(($1..$2)[9,0,15,7]):e
this is just a regex substitution with a bit of perlcode in it (the "e" at the end says perl to "Evaluate the right side as an expression.").
ok, the left side captures "A" and "(" from the string given from a hashref in scalar context (remember?). now the right side generates a list by "A" .. "(" which seems to "realy" generate a list from "A" to "Z" (is this documented somewhere?). from this list the elemnts "[9,0,15,7]" are given to print. thats it!
easy, hu?