... what does the following mean?

$value =~ s/%([a-fA-F0-9][a-fA-F0-9])/pack("C", hex($1))/eg;
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It decodes url-encoded text. Ie. When you see url's that look have characters that have been encoded, eg. http://news.bbc.co.uk/1/hi/technology/default%20.stm, it looks for the %xx encoding and translates that back to an ascii character.

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
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