Kevin's Weblog

I haven’t posted a Hall of Shame entry in a while, and this one adds a new twist. Today, it’s perl itself that gets the blame. (I know, some will say that’s always the case.) This is about when two magics collide.

There are a couple magic tokens that you can use in perl scripts to represent the current context, like __PACKAGE__ and __LINE__. These tokens behave differently from variables, and one example of this is that they don’t interpolate inside strings.

For reasons of backwards compatibility, when perl encounters a bareword that it doesn’t recognize, it ‘auto-quotes’ it (treats it as a string). This is generally considered A Bad Thing and the strict pragma forbids it. Except for when it doesn’t, and that exception is hash keys. So, even with strict, a bareword hash key is magically auto-quoted.

Now, consider this code:

package foo;
use strict;
my %h; 
$h{__PACKAGE__} = "bar"; 
while (my ($k, $v) = each %h) { print "$k => $v" };

I think it’s clear that anyone writing this code expects the output to be

foo => bar

Unfortunately, it’s actually

__PACKAGE__ => bar

What’s happened is that perl has auto-quoted the string before doing the token substitution. In order to get the desired behavior, it’s necessary to resort to atrocities like

$h{__PACKAGE__ . ""} = "bar"; 

or

$h{+__PACKAGE__} = "bar"; 

to get the job done

The switch to feedback allowed me to easily close comments on articles after a couple months. This is the single most effective anti-spam measure I’ve tried. Before this change, I was getting about 300 spam comments a day, with Akismet achieving 98-99% effectiveness. This was leaving me with a handful of spams that I had to clean up manually each day.

With feedback preemptively rejecting any comment submissions on old posts, I now only get 1-2 spams a day that are submitted to Akismet, and it has caught all of them so far.

As an aside, this seems to suggest a shortcoming in Akismet’s learning algorithms. The Akismet query API includes the article permalink, and almost all of my spam was on 3 or 4 articles. Since I was submitting feedback to Akismet on each of the spams it missed, it really should have learned that comments on those articles were spam with extremely high probability. On the contrary, though, I never observed a decline in the false negative rate on those articles.