It will fail on this:

my $increment_by_secret = do {
  my $secret_number = 7;
  sub { $secret_number + shift };
};
[download]

It fails because there's no access to $secret_number from outside the coderef, so we can't tell what the "magic constant" should be. All you get is:

{
    ($secret_number + shift(@_));
}
[download]

(Just tested.)

-- Randal L. Schwartz, Perl hacker