So, ErosBlog has a long-in-time but small-in-number history of posts about people who stick beads (or other small objects) under the skin of their penises:

Lumpy Spotted Dick

The Feel Of The Bumpy Cock
Beads Inserted For Her Pleasure

If you look at those posts, you’ll find commenters chiming in with various additional examples and cultural accounts describing or attempting to explain the practice. For instance:

Yakuza [Japanese gangsters] have a long tradition of inserting pearls neath the skin of the penis. When in gaol, they slit one in for each year of incarceration.

The nifty thing about (sub)cultural practices, though, is the way they tend to travel. Imagine my bemusement at discovering this link (via Tiny Nibbles, thanks Violet!):

Subcutaneous Penile Insertion of Domino Fragments by Incarcerated Males in Southwest United States Prisons: A Report of Three Cases

In each case, an incarcerated Hispanic male or fellow inmate filed a domino into a unique shape for placement under the penile skin. Utilizing the tip of a ballpoint pen or a sharpened shard of plastic to create a puncture wound, each man inserted the domino fragment into the subcutaneous tissue of the penis…