$value =~ tr/+/ /;
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replaces + characters with spaces

$value =~ s/%([a-fA-F0-9][a-fA-F0-9])/pack("C",hex($1))/eg;
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replaces pairs of hex digits preceded by % with the character that has that value. The /e flag causes pack("C",hex($1)) to be executed and the result is used for the substitution string. For example '%41' would become 'A' (assuming ASCII is being used).