Google says http://www.cgi101.com/class/ch4/text.html

Next is a rather complicated regular expression that substitutes every %HH hex pair back to its equivalent ASCII character, using the pack() function. We'll learn exactly how this works in Chapter 14, but for now we'll just use it to parse the form data:

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