my $sql = "SELECT * FROM tbl_admin WHERE day = '$d' ....";
my $sth = $dbh->prepare($sql);
$dbh->exec();

##</code><code>##

my $sql = "SELECT * FROM tbl_admin WHERE day = ? ....";
# note the ? (question mark) placeholder
my $sth = $dbh->prepare($sql);
$dbh->exec($d);