1) If you wish to emphasize on "Jackson" So far, only Jackson has placed an order. You do not have to use a verbal. A verbal is basically a verb, that is not a verb. It is the form of a verb that can function as a noun, an adjective, or an adverb. There are three types of verbals: - infinitives; - participles; and - gerunds. An infinitive is a verb form preceded by the particle "to". As there are three types of verbals, there are three types of verbal phrases: - infinitive phrases; - participial phrases; and - gerund phrases. An infinitive phrase starts with an infinitive and is followed by objects and/or modifiers. 2) As the writer probably wanted to emphasize on Jackson being "the only person", the writer used an infinitive phrase (to have placed an order) to modify "the only person". The writer used the present perfect tense verb form. So far, Jackson, is the only person to have placed an order.

The phrase "to have placed" acts as a verbal in the form of an infinitive. The phrase modifies "person". You cannot substitute "have placed", because it would form an out-of-place clause. It would be non-sensical.