This is not something that is specific to a verb of perception. You can use any transitive verb in a restrictive clause where the object of that verb is the noun that the restrictive clause modifies:
(A) I sent the letter.
(B) This is the letter that I sent.
In (A), sent is a transitive verb with [the] letter as its object. In (B), the bolded text is a restrictive clause modifying the noun letter. Note that the transitive verb sent in (B) has no object: it refers back to the noun that the phrase is modifying..
Verbs of perception are no different than ordinary verbs - they can be used in a restrictive clause to modify a noun, which acts like the object of that verb of perception. The -ing verb can either go with the object or stay within the clause:
(A) I have seen the man washing his car.
(B) He is the man that I have seen washing his car.
(C) He is the man washing his car that I have seen.
Note that both (B) and (C) are grammatical and have a slightly different emphasis, where the emphasis is on whatever is last in the sentence. (B) emphasizes the action of the man washing his car, while (C) emphasizes my seeing the man.