「Throughout much of recorded history, a natural pearl necklace comprised of matched spheres was a treasure of almost incomparable value, in fact the most expensive jewelry in the world. …Before the creation of cultured pearls in the early 1900s, natural pearls were so rare and expensive that they were reserved almost exclusively for the noble and very rich. A jewelry item that today's working women might take for granted, a 16-inch strand of perhaps 50 pearls, often costs between $500 and $5,000. At the height of the Roman Empire, when pearl fever reached its peak, the historian Suetonius wrote that the Roman general Vitellius financed an entire military campaign by selling just one of his mother's pearl earrings.」
[ https://www.pbs.org/wgbh/nova/article/history-pearls/ 2022-01-21 ] therefore, inserting actual pearls into the mouth of a corpse at burial seems to be a terrible waste (and not likely) due to the extreme cost of pearls in the old days. Nonetheless,「stuffing long strings of pearls into an orifice」(one without teeth) and then pulling out the string slowly while in contact with the clitoris is a distinct possibility, an exercise performed by bored and horny harem concubines for personal pleasure (after their old lord's manhood had died, or, when scheduled late in the concubine rotation), a practice rumored still to be viewable today via the Internet as is most anything else imaginable.
Apparently this custom is still extant, although some would have us believe otherwise: perhaps evolved from the age-old practice of emasculating a vanquished enemy and stuffing his jewels both into and protruding from the mouth of his decapitated head while on display for propaganda purposes (what humans do…):「rulership to bear in mind and understand」the power of the victorious king…old customs die hard. Note that the Japanese pronunciation literally means「stuffed balls.」
「Stuffing pearls into an orifice」can be better understood when one recalls the euphemism in dynastic China that likens pearls to drops of ejaculated semen. PEARLS and「sources of pearls」used in Chinese euphemistically that were「stuffed into an orifice of a corpse」suggests farewell necrophilia access to any and all orifices of the deceased, perhaps leaving only fond memories.
「Royal tendencies with mouth orifice」for inserting 'pearls' of ejaculated semen into a mouth is suggestive of「sucking off the king」(another rendition of these elements) as a job one has no choice but to perform even if the lord is infected, and which might lead to an early grave for the sucker, too: faithfully performing fateful services on a「rod with one foot in the grave.」

External source definitions …excluding politically incorrect concepts and other meanings deemed offensive today; may include extended definitions, euphemisms, and mnemonics)
Chinese: gems or pearls formerly put into the mouth of a corpse
Japanese EDRDG: gems of pearls formerly put into the mouth of a corpse
Unihan extended: gems of pearls formerly put into the mouth of a corpse