One way is to put in a subroutine to create the inner hash for you. If you prefer to have everything located inside the map, thats easy using an anonymous sub:

my %hash = 
    map {$_ => {
                sub { 
                     key1 => $_[0],
                     key2 => alter($_[0])
                    }->('longstring'.$_.'more')
               }
    } @list;
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To answer your original question, in short: no. Before its initialized there is nothing there to use.

Basically you assign a list to the hash, and that list has to be constructed before it can be assigned, and its while you are constructing this list that you want to use one of its earlier elements to create a later one. Using a sub, you can create the element in question first(when providing the arguments to the sub call), and then create the list.