One problem with using dynamic scoping is that you've opened up the possibility that the same variable could be used somewhere else for a different purpose. That is the advantage to using lexicals.

Another solution is to use ReleaseAction to do cleanup when you exit the sub.

sub outer {
  my $helper;
  $helper = sub {
    ...
  };
  my $delay_cleanup = on_release {$helper = undef};
  $helper->(@_);
}
[download]

Effectively it does the same thing as the weaken solution, but the code is a little more explicit about what is happening, when. I'd personally lean towards this solution.

And a final option. Larry thinks that using dynamic scoping on lexical variables is too confusing to allow. But he does allow it for data structures. Which leads to the convoluted:

sub outer {
  my @helper;
  local $helper[0] = sub {
    ...
  };
  $helper[0]->(@_);
}
[download]

I wouldn't suggest this solution for fear of the psychopathic maintenance programmer demanding to know why this works. But every year or two I wind up using this fact in a recursive function to detect and track down potential deep recursion bugs. For instance I'm recursing through a set of nodes and I'll do something like this:

{
  my %in_node;
  sub something_recursive {
    my $node = shift;
    if ($in_node{$node->{name}}) {
      confess("Can't access $node->{name} while accessing $node->{name
+}");
    }
    local $in_node{$node->{name}} = 1;
    ...
  }
}
[download]

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