muba: good stuff.
You may wish to consider the following (mostly minor and occasionally open to debate) re the idiom or syntax:
In Introduction, "Note: this is not a regex tutorial or regex howto." (emphasis supplied) s/or /nor/
likewise, s/If you may ever find /If you ever find / (
Jargon: "Before I finnaly start off, let's set some terminology." -- for spelling change to "finally"; for idiom: just omit it entirely.
Rules of Thumb 2: I'm intruding into content here, but I'm troubled by the statement, "when input from external sources may be unsafe." My view: input from external sources is ALWAYS unsafe... even if it's coming from me. No malice is required: "Fat fingeritis" can wreak havoc!
RoT 2: "...etc) is considered 'tainted'." s/is/are/ for subject-verb agreement in quantity;
also in RoT 2, for brevity: "There are several ways to untaint data, which I am not about to mention here. You should check the above mentioned Perl Security (perlsec) manpage." could be written, "There are several ways to untaint data for which you should check the above-mentioned...."
RoT3: "They are ignorant or else they are malicious." would be less globally applicable to (all) users) if you said, "Some are ignorant; some are malicious." (As written, the current phrase indicts ALL users.)
RoT 5. "syntaxis" -- I think you want "syntax" and
RoT 6. spelling: s/shuld/should/
RoT 7. "Do use CGI; (have..." might be clearing if you were to say "Do use CGI:; (have..." or, even better, if you specified the module by its full name
If you find these useful (msg me), I'll carry on with the rest of the document.