MidLifeXis was so kind as to recommend my module DBIx::PreQL. It's aim is to simplify the horrendous task of maintaining dynamically generated SQL.

Here's one way to handle the your particular case.

my $preql = <<'PREQL'
 *      SELECT
 *          Name
 *      FROM
 CUST       Customer
 SALE       Sales
 *      WHERE
 CUST      CustID = ?customer_id?
 SALES     SalesID = ?sales_id?     
PREQL

my ($query, @params) = DBIx::PreQL->build_query(
     query => $preql,
     data => {
        sales_id => $salesid,
        customer_id => $custid,
     },
    wanted => $x > 10 ? 'CUST' : 'SALES'
 );

my $got = $db->selectall_hashref( $query, @params );
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Why do all this stuff?

• You get the placeholders you wanted for your dynamic query.
• This is the least sucky way I've found to build dynamic SQL. The code I maintain has tons of dynamic SQL and I spent years experimenting before I hit upon this approach.
• You will get a good error message if you put yourself in a condition where you are expecting a customer id but one isn't available.

In fact, at $WORK, we have found it worthwhile to convert static queries to PreQL. It has helped with debugging and maintenance. Because all the placeholders are labeled, they are easier to understand. PreQL also provides useful error messages that identify missing, but expected placeholder values.