For completeness sake...

I understand what each line does ... for example ...

$value =~ tr/+/ /;
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performs transliteration on $value and thus translates characters like !, @, #, etc. to their hex equivalents ... that is, # becomes %23.

No, all it does is replace plus characters with spaces.

$value =~ s/%(..)/pack("c", hex($1))/ge;
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The line above then takes the two "hex" digits that make up the string and packs them into their hex equivalent.

And this does the opposite of what you thought the first line does, i.e. it replaces things like %23 with # etc. (Perhaps that's what you were saying but I couldn't quite tell from your wording)