my ( $query, @args );
if ( $x > 10 ) {
    $query = 'select Name from Customers where CustID = ?';
    @args = ( $custid );
} else {
    $query = 'select Name from Sales where SalesId = ? and CustID = ?';
    @args = ( $salesid, $custid );
}

my $sth = $dbh->prepare( $query );
$sth->execute( @args );