Learn Portuguese (Portugal): Quem a viu limpar a lágrima foi o encenador. - Who saw her wipe the tear was the stage director.

This is a great example of clitic climbing with a perception verb (ver). The pronoun a ("her") represents the person doing the wiping, but it climbs up to attach to the main verb (viu). Additionally, the word Quem triggers proclisis, forcing the pronoun to come before viu (Quem a viu) rather than attaching after it with a hyphen.

Is a the object of viu or the subject of limpar?

Logically, she is the subject of limpar because she is the one wiping the tear. However, grammatically, a acts as the direct object of the perception verb viu (he saw her). This dual role is exactly what makes perception verbs (like seeing and hearing) and causative verbs (like making and letting) special when they are followed by an infinitive.

Could I rewrite this without the Quem ... foi ... structure?

Yes! The standard word order would be O encenador viu-a limpar a lágrima. The Quem ... foi ... structure is used to add strong emphasis, highlighting that it was specifically the stage director, and no one else, who saw her do it.

Why does the Portuguese sentence say a lágrima (the tear) instead of a sua lágrima (her tear)?