my $uname = 'leander@whatever.com';
my $sql = 'SELECT uname, pwd FROM login WHERE uname=' . $uname;
warn $sql; # <====== would print:
           # SELECT uname, pwd FROM login WHERE uname=leander@whatever.com
           # which is illegal SQL
my $res = $dbh->selectall_arrayref($sql) || die $dbh->errstr;