...
$dbh->{AutoCommit} = 0; # disable auto-commit

my $sth_insert_login = $dbh->prepare(<<'SQL');
    insert into logintbl(
           username, password)
    values(?,        ?)
SQL

my $sth_insert_student = $dbh->prepare(<<'SQL');
    insert into studenttbl(
            fname,lname,grade,DOB,email,tel_no,membertype,username)
    values (?,    ?,    ?,    ?,  ?,    ?,     ?,         ?)
SQL
#$dbh->do ("insert into logintbl(username, password) values('jones2100','password')");
$sth_insert_login->execute('jones2100', 'password');

#$dbh->do ("insert into studenttbl(fname,lname,grade,DOB,email,tel_no,membertype,username) VALUES ('jamie','batty','7thkyu','1988/07/21','battydave&#64hotmail.com','01234523454','monthly pay','jones2010')");
$sth_insert_student->execute('jamie','batty','7thkyu','1988/07/21','battydave@hotmail.com','01234523454','monthly pay','jones2010');

$dbh->commit();