Prematurely breaking a select can be just as bad as inserting additional statements; consider this:

my ($id) = $dbh->selectrow_array("SELECT id FROM users WHERE NAME='$na
+me' AND PASSWORD='$password'");
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If I somehow manage to break the SQL statement in $name, my password will not be checked! Without a good quoting mechanism, you could break that with

$name = "admin\'\0";
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or even

$name = "admin';";
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Or whatever kind of escapes your specific DBD/database combination will allow. This is the main reason for using $dbh->quote() and placeholders - the quoting mechanism can be different for different databases, and they are a little more complex than you imagine. In effect, all you're doing is trying to reinvent the $dbh->quote() method.

Why reinvent the wheel when there already is one that's been especially made for your type of car, has been checked and double-checked, and is already safely attached to your car?