Lead is going to be absorbed into the leather project if it comes in contact with a lead block. That means anyone handling the project afterwards will be coming in contact with the contaminants. Lead exposure occurs through inhalation, ingestion and absorption through the skin. Children and pregnant women are at higher risk then men, but lead can accumulate and be stored in your body for up to 30 years. Using a lead block when punching holes is likely to only be a minor exposure concern, however, if you do enough of it, over time you could end up with a significant chronic lead exposure. After spending 30 years in the hazardous materials consulting field, I would discourage using a lead block as a base for leather working.
IMO, a softwood stump, or better yet a soft hardwood stump, such as a large diameter piece of aspen firewood, placed on end, with a piece of heavy leather on top of it is the best choice.