$_="10101010000011110101000101001110110010100111";

(reverse, #I need it reversed so what follows will work
s/(.)(.)(.)(.)/ #get the four digits && store them
$a+=2**($_-1)if$$_/ee #take two to the $_power if
                      #$$_ exists(e.g., $1 set, $2 set,etc)
for 1..4 &&           #do it for each digit and
s/1(?=\d)// && y/0-5/a-f/)  #if 2-digit answer convert
                            #to appropriate letter for hex
for split /(....)/;         #do this for all 4 digits
                            #in $_
print $a, "\n";