RIGHT:
my $sth = $dbh->prepare(
    "UPDATE foo SET bar=7 WHERE baz=?"
);
for my $val(@values) {
    $sth->execute($val);
}

WRONG:
for my $val(@values) {
    $val = $dbh->quote($val);
    my $sth = $dbh->do(
       "UPDATE foo SET bar=7 WHERE baz=$val"
    );
}