The following code returns 14 as you'd expect:

Block[{expr},
  expr = 2 z;
  f[z_] = expr;
  f[7]]

But if you change that Block to a Module then it returns 2*z. It seems to not matter what other variables besides expr you localize. I thought I understood Module, Block, and With in Mathematica but I can't explain the difference in behavior between Module and Block in this example.

Related resources:

• Tutorial on Modularity and the Naming of Things from the Mathematica documentation
• Excerpt from a book by Paul R. Wellin, Richard J. Gaylord, and Samuel N. Kamin
• Explanation from Dave Withoff on the Mathematica newsgroup