I suspect your definition of a "placeholder" is my definition of a "host variable".
They are essentially the same thing. Though when I think of 'host variables' (or bind variables), it's usually in languages that support embedding a named variable inside the sql statement, e.g. "select stuff from table where id_column = :id_number", and 'id_number' is a variable in the program. The Perl/DBI way is to use a question mark instead of a program variable (which is what many databases natively support anyway), then bind a value to it when (or before) you execute the statement. (some databases support named parameters which would look like a host variable, but in perl you'd still have to bind a value to it, so it wouldn't be an actual perl 'host' variable).
Placeholders, bind variables, host variables, or whatever you want to call them are a good idea in any programming language database API that supports them, and pasting raw unfiltered (and untrusted) user input into a SQL statement in any programming language is bad.
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