Yes, stacks of nested subs are super-cool. But, uh, why bother writing all recursive code this way? I don't see any gain in efficiency (if anything, creating all those subs might slow things down a tad), and it introduces an extra level of "WTF?" to people reading the code.

On the other hand, if you're writing tail-recursive code, this is a great way of not exposing an accumulator variable to the rest of the world. For example (untested):

sub fact
{
  my $fact_tr; # needs to exist before it gets assigned to
  $fact_tr = sub {
    my ($v, $r) = @_;
    return 1 if $v < 2; # oversimplified
    $fact_tr->($v-1, $v * $r);
  }; # thanks ihb for pointing out the missing semi
  $fact_tr->(@_, 1);
}
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Edit: Separated declaration and use of $fact_tr. Thanks ihb!