i'm not a master of DBI nor a Taint one, but you should understand the distinction between them. you MUST use placeholders when DBI is involved. is a good practice should never be avoided even if you are the only user of the application, in my opinion and experience.
is whole another story. it assume that all, and i say all, input coming from outside the source code of your Perl program is evil. Evilness is viral so if you mix presumed-evil-data with other data the result is another evil-presumed data. Taint mode is explained also in
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There are no rules, there are no thumbs..
Reinvent the wheel, then learn The Wheel; may be one day you reinvent one of THE WHEELS.