Larding needle

A larding needle is a tool used to pierce a piece of meat and insert strands of fat to provide moisture and flavor to lean meats and wild fowl.  This tool is typically made from stainless steel and may or may not have a wooden handle. 

Look for this product at butcher supply stores or it is available online at Lehman's.

Alternately you could "bard" the meat, wrap it in a thin layer of fat