There is nothing wrong with putting plastic over the shroud, to stop blood and unpleasant smells coming out. But if perfume is enough to stop the unpleasant smell, then you should stick to using that.
Shaykh Ibn Baaz (may Allaah have mercy on him) was asked: Some of those who wash the dead put a plastic bag over the one who has died in a car accident so that the blood will not come out onto the shroud.
There is nothing wrong with putting something over the wound to seal it. End quote from Majmoo’ Fataawa al-Shaykh Ibn Baaz (13/128).
And Allaah knows best.