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She sprayed perfume on the dress.
Literal
She [topic-は] dress [onto-に] perfume [object-を] sprinkled.
振り掛ける fuses 振る (shake) with 掛ける (apply over), giving 'sprinkle/spray onto.' The に marks where the perfume lands. The same verb sits behind the noun ふりかけ — those rice toppings of dried fish, seaweed, and seasoning that you sprinkle over a bowl. Recognizing compound verbs like this lets you guess at related vocabulary you've never seen before.