$value =~ s/%([a-fA-F0-9][a-fA-F0-9])/pack("C", hex($1))/eg;

##</code><code>##

s/
    %                             ## find (and discard) the % char
   (                              ## capture 
       [a-fA-F0-9][a-fA-F0-9]     ## two hex charcters to $1
   )
/
    pack("C", hex($1))            ## convert hexcharacters to a number, 
                                  ## then pack to a character
/eg;  ## replace all in the target string