How do I decode or create those %-encodings on the web?

If you are writing a CGI script, you should be using the CGI.pm module that comes with perl, or some other equivalent module. The CGI module automatically decodes queries for you, and provides an escape() function to handle encoding.

The best source of detailed information on URI encoding is RFC 2396. Basically, the following substitutions do it:

    s/([^\w()'*~!.-])/sprintf '%%%02x', ord $1/eg;   # encode

    s/%([A-Fa-f\d]{2})/chr hex $1/eg;            # decode

However, you should only apply them to individual URI components, not the entire URI, otherwise you'll lose information and generally mess things up. If that didn't explain it, don't worry. Just go read section 2 of the RFC, it's probably the best explanation there is.

RFC 2396 also contains a lot of other useful information, including a regexp for breaking any arbitrary URI into components (Appendix B).

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