Proc for fully decoding URI's

If I may add, the URI::decode command is actually a string-level parser. You could just as easily use it like this:

when RULE_INIT {
    set uri "/something%20something%20something/blah"
    log local0. [URI::decode $uri]
}

What you couldn't do is use a protocol-level command, like HTTP::uri, inside a proc. Otherwise a proc is just a function that takes inputs and returns an output. There are of course going to be exceptions to this rule, but you can mostly say that any command that's prefixed with a specific protocol (ie. HTTP::, DNS::, SSL::, TCP::, etc.) isn't going to be directly accessible inside a proc. Anything else is either fair game or can be passed in as a parameter.