my %query_map; # Actually declared in higher (more outer) scope.
if ( $x > 10 ) {
    $query = "SELECT Name FROM Customers WHERE CustId = ?";
}
else {
    $query = "SELECT Name FROM Sales WHERE SalesId = ?";
}
$query_map{$query} ||= $dbh->prepare($query);

$query_map{$query}->execute(@args_that_will_match_query);