I'd actually start from the other end, looking for infrequently-occuring-but-nonunique lines. A line which is used in exactly 2 or 3 places is more likely to be part of a block of cut and paste than one used in 100.

That's a great suggestion, I hadn't thought of that yet. Thanks!

I'm well aware of the various refactoring techniques, and I do use both OO and functional approaches where appropriate. Your example is illustrative enough though, although your walk_tree should probably read:

sub walk_tree
{
  my $tree = shift;
  my $per_node = shift;
  walk_tree($tree->left);
  $per_node->($tree->node);
  walk_tree($tree->right);
}
[download]