Yikes, that's a bug(update: shows a bug in perl itself). The call to length should explicitly be passing $_, but it's not (update: passes nothing) when it is overridden. This causes a problem with lexical $_:

$ perl5.9.4 -w
BEGIN{
  *CORE::GLOBAL::length = sub (;$) {
    my $arg = @_ ? $_[0] : $_;
    defined($arg) ? CORE::length($arg) : undef;
  }
}
$_ = "abc";
my $_;
$_ = "defghi";
print length;
__END__
3
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This should print 6 whether length is overridden or not, but as is prints 3 when it is overridden.

Update: after consideration, I think this is just a natural consequence of the ;$ prototype, and should be no longer a problem with a _ prototype (when someone implements that).